Omit User-Agent: header by default

The User-Agent: header can be fun and interesting, but it also leaks
quite a bit of information about the user and their software stack.

This represents a potential security risk (attackers can target the
particular stack) and also an anonymity risk (a user trying to
preserve their anonymity by sending mail from a non-associated account
might reveal quite a lot of information if their choice of mail user
agent is exposed).

This change also avoids hiding the User-Agent header by default, so
that people who decide they want to send it will at least see it (and
can edit it if they want to) before sending.

It makes sense to have safer defaults.

emacs/notmuch-mua.el

@@ -62,7 +62,7 @@ disabled: this would result in an incorrect behavior."))
 		 (const :tag "Compose mail in a new window"  new-window)
 		 (const :tag "Compose mail in a new frame"   new-frame)))
 
-(defcustom notmuch-mua-user-agent-function 'notmuch-mua-user-agent-full
+(defcustom notmuch-mua-user-agent-function nil
   "Function used to generate a `User-Agent:' string. If this is
 `nil' then no `User-Agent:' will be generated."
   :type '(choice (const :tag "No user agent string" nil)
@@ -73,7 +73,7 @@ disabled: this would result in an incorrect behavior."))
 			   :value notmuch-mua-user-agent-full))
   :group 'notmuch-send)
 
-(defcustom notmuch-mua-hidden-headers '("^User-Agent:")
+(defcustom notmuch-mua-hidden-headers nil
   "Headers that are added to the `message-mode' hidden headers
 list."
   :type '(repeat string)

test/T310-emacs.sh

@@ -193,7 +193,6 @@ emacs_deliver_message \
      (kill-whole-line)
      (insert "To: user@example.com\n")'
 sed \
-    -e s',^User-Agent: Notmuch/.* Emacs/.*,User-Agent: Notmuch/XXX Emacs/XXX,' \
     -e s',^Message-ID: <.*>$,Message-ID: <XXX>,' \
     -e s',^\(Content-Type: text/plain\); charset=us-ascii$,\1,' < sent_message >OUTPUT
 cat <<EOF >EXPECTED
@@ -201,7 +200,6 @@ From: Notmuch Test Suite <test_suite@notmuchmail.org>
 To: user@example.com
 Subject: Testing message sent via SMTP
 Date: 01 Jan 2000 12:00:00 -0000
-User-Agent: Notmuch/XXX Emacs/XXX
 Message-ID: <XXX>
 MIME-Version: 1.0
 Content-Type: text/plain
@@ -310,7 +308,6 @@ test_emacs '(let ((message-hidden-headers ''()))
 	    (test-output))'
 sed -i -e 's/^In-Reply-To: <.*>$/In-Reply-To: <XXX>/' OUTPUT
 sed -i -e 's/^References: <.*>$/References: <XXX>/' OUTPUT
-sed -i -e 's,^User-Agent: Notmuch/.* Emacs/.*,User-Agent: Notmuch/XXX Emacs/XXX,' OUTPUT
 cat <<EOF >EXPECTED
 From: Notmuch Test Suite <test_suite@notmuchmail.org>
 To: user@example.com
@@ -318,7 +315,6 @@ Subject: Re: Testing message sent via SMTP
 In-Reply-To: <XXX>
 Fcc: ${MAIL_DIR}/sent
 References: <XXX>
-User-Agent: Notmuch/XXX Emacs/XXX
 --text follows this line--
 Notmuch Test Suite <test_suite@notmuchmail.org> writes:
 
@@ -335,7 +331,6 @@ test_emacs "(let ((message-hidden-headers '()))
 	    (notmuch-test-wait)
 	    (notmuch-search-reply-to-thread)
 	    (test-output))"
-sed -i -e 's,^User-Agent: Notmuch/.* Emacs/.*,User-Agent: Notmuch/XXX Emacs/XXX,' OUTPUT
 cat <<EOF >EXPECTED
 From: Notmuch Test Suite <test_suite_other@notmuchmail.org>
 To: Sender <sender@example.com>
@@ -343,7 +338,6 @@ Subject: Re: ${test_subtest_name}
 In-Reply-To: <${gen_msg_id}>
 Fcc: ${MAIL_DIR}/sent
 References: <${gen_msg_id}>
-User-Agent: Notmuch/XXX Emacs/XXX
 --text follows this line--
 Sender <sender@example.com> writes:
 
@@ -361,7 +355,6 @@ test_emacs "(let ((message-hidden-headers '()))
 	    (notmuch-test-wait)
 	    (notmuch-search-reply-to-thread)
 	    (test-output))"
-sed -i -e 's,^User-Agent: Notmuch/.* Emacs/.*,User-Agent: Notmuch/XXX Emacs/XXX,' OUTPUT
 cat <<EOF >EXPECTED
 From: Notmuch Test Suite <test_suite@notmuchmail.org>
 To: Sender <sender@example.com>, someone@example.com
@@ -369,7 +362,6 @@ Subject: Re: ${test_subtest_name}
 In-Reply-To: <${gen_msg_id}>
 Fcc: ${MAIL_DIR}/sent
 References: <${gen_msg_id}>
-User-Agent: Notmuch/XXX Emacs/XXX
 --text follows this line--
 Sender <sender@example.com> writes:
 
@@ -382,7 +374,6 @@ test_emacs '(let ((message-hidden-headers ''()))
 	    (notmuch-show "id:20091118002059.067214ed@hikari")
 		(notmuch-show-reply)
 		(test-output))'
-sed -i -e 's,^User-Agent: Notmuch/.* Emacs/.*,User-Agent: Notmuch/XXX Emacs/XXX,' OUTPUT
 cat <<EOF >EXPECTED
 From: Notmuch Test Suite <test_suite@notmuchmail.org>
 To: Adrian Perez de Castro <aperez@igalia.com>, notmuch@notmuchmail.org
@@ -390,7 +381,6 @@ Subject: Re: [notmuch] Introducing myself
 In-Reply-To: <20091118002059.067214ed@hikari>
 Fcc: ${MAIL_DIR}/sent
 References: <20091118002059.067214ed@hikari>
-User-Agent: Notmuch/XXX Emacs/XXX
 --text follows this line--
 Adrian Perez de Castro <aperez@igalia.com> writes:
 
@@ -447,7 +437,6 @@ test_emacs '(let ((message-hidden-headers ''()))
 	    (notmuch-show "id:cf0c4d610911171136h1713aa59w9cf9aa31f052ad0a@mail.gmail.com")
 		(notmuch-show-reply)
 		(test-output))'
-sed -i -e 's,^User-Agent: Notmuch/.* Emacs/.*,User-Agent: Notmuch/XXX Emacs/XXX,' OUTPUT
 cat <<EOF >EXPECTED
 From: Notmuch Test Suite <test_suite@notmuchmail.org>
 To: Alex Botero-Lowry <alex.boterolowry@gmail.com>, notmuch@notmuchmail.org
@@ -455,7 +444,6 @@ Subject: Re: [notmuch] preliminary FreeBSD support
 In-Reply-To: <cf0c4d610911171136h1713aa59w9cf9aa31f052ad0a@mail.gmail.com>
 Fcc: ${MAIL_DIR}/sent
 References: <cf0c4d610911171136h1713aa59w9cf9aa31f052ad0a@mail.gmail.com>
-User-Agent: Notmuch/XXX Emacs/XXX
 --text follows this line--
 Alex Botero-Lowry <alex.boterolowry@gmail.com> writes:
 
@@ -521,7 +509,6 @@ test_emacs "(let ((message-hidden-headers '()))
 	    (notmuch-show \"id:${gen_msg_id}\")
 	    (notmuch-show-reply)
 	    (test-output))"
-sed -i -e 's,^User-Agent: Notmuch/.* Emacs/.*,User-Agent: Notmuch/XXX Emacs/XXX,' OUTPUT
 cat <<EOF >EXPECTED
 From: Notmuch Test Suite <test_suite@notmuchmail.org>
 To: 
@@ -529,7 +516,6 @@ Subject: Re: Reply within emacs to an html-only message
 In-Reply-To: <${gen_msg_id}>
 Fcc: ${MAIL_DIR}/sent
 References: <${gen_msg_id}>
-User-Agent: Notmuch/XXX Emacs/XXX
 --text follows this line--
 Notmuch Test Suite <test_suite@notmuchmail.org> writes:
 
@@ -546,7 +532,6 @@ test_emacs "(let ((message-hidden-headers '()))
 	      (notmuch-show \"id:$message_id\")
 	      (notmuch-show-reply)
 	      (test-output))"
-sed -i -e 's,^User-Agent: Notmuch/.* Emacs/.*,User-Agent: Notmuch/XXX Emacs/XXX,' OUTPUT
 cat <<EOF >EXPECTED
 From: Notmuch Test Suite <test_suite@notmuchmail.org>
 To: 
@@ -554,7 +539,6 @@ Subject: Re: Quote MML tags in reply
 In-Reply-To: <test-emacs-mml-quoting@message.id>
 Fcc: ${MAIL_DIR}/sent
 References: <test-emacs-mml-quoting@message.id>
-User-Agent: Notmuch/XXX Emacs/XXX
 --text follows this line--
 Notmuch Test Suite <test_suite@notmuchmail.org> writes: