The copying has nothing to do with stdin, so call it copy_fd
instead. While at it, improve documentation and reverse the
parameters, as destination is traditionally the first parameter.
An "is something" predicate conveys the meaning better. While at it,
improve the function documentation and error message. Besides the
error message change, no functional changes.
Apart from the status codes for format mismatches, the non-zero exit
status codes have been arbitrary. Make the cli consistently return
either EXIT_SUCCESS or EXIT_FAILURE.
The notmuch insert command reads a message from standard input,
writes it to a Maildir folder, and then incorporates the message into
the notmuch database. Essentially it moves the functionality of
notmuch-deliver into notmuch.
Though it could be used as an alternative to notmuch new, the reason
I want this is to allow my notmuch frontend to add postponed or sent
messages to the mail store and notmuch database, without resorting to
another tool (e.g. notmuch-deliver) nor directly modifying the maildir.
