lib: remove enum names from typedefs

There are some enum typedefs with the enum name:

    typedef enum _name_t { ... } name_t;

We don't need or use the enum names _name_t for anything, and not all
of the enum typedefs have them. We have the typedefs specifically to
use the typedef name.

Use the anonymous enum in the typedefs:

    typedef enum { ... } name_t;

lib/database-private.h

@@ -160,7 +160,7 @@ operator&= (_notmuch_features &a, _notmuch_features b)
 
 /*
  * Configuration options for xapian database fields */
-typedef enum notmuch_field_flags {
+typedef enum {
     NOTMUCH_FIELD_NO_FLAGS	= 0,
     NOTMUCH_FIELD_EXTERNAL	= 1 << 0,
     NOTMUCH_FIELD_PROBABILISTIC = 1 << 1,

lib/notmuch-private.h

@@ -121,7 +121,7 @@ typedef enum {
  */
 #define NOTMUCH_MESSAGE_ID_MAX (200 - sizeof (NOTMUCH_METADATA_THREAD_ID_PREFIX))
 
-typedef enum _notmuch_private_status {
+typedef enum {
     /* First, copy all the public status values. */
     NOTMUCH_PRIVATE_STATUS_SUCCESS			= NOTMUCH_STATUS_SUCCESS,
     NOTMUCH_PRIVATE_STATUS_OUT_OF_MEMORY		= NOTMUCH_STATUS_OUT_OF_MEMORY,
@@ -173,7 +173,7 @@ typedef enum _notmuch_private_status {
      (notmuch_status_t) private_status)
 
 /* Flags shared by various lookup functions. */
-typedef enum _notmuch_find_flags {
+typedef enum {
     /* Lookup without creating any documents.  This is the default
      * behavior. */
     NOTMUCH_FIND_LOOKUP = 0,

lib/notmuch.h

@@ -112,7 +112,7 @@ typedef int notmuch_bool_t;
  * A zero value (NOTMUCH_STATUS_SUCCESS) indicates that the function
  * completed without error. Any other value indicates an error.
  */
-typedef enum _notmuch_status {
+typedef enum {
     /**
      * No error occurred.
      */
@@ -1678,7 +1678,7 @@ notmuch_message_reindex (notmuch_message_t *message,
 /**
  * Message flags.
  */
-typedef enum _notmuch_message_flag {
+typedef enum {
     NOTMUCH_MESSAGE_FLAG_MATCH,
     NOTMUCH_MESSAGE_FLAG_EXCLUDED,
 
@@ -2524,7 +2524,7 @@ notmuch_config_list_destroy (notmuch_config_list_t *config_list);
 /**
  * Configuration keys known to libnotmuch
  */
-typedef enum _notmuch_config_key {
+typedef enum {
     NOTMUCH_CONFIG_FIRST,
     NOTMUCH_CONFIG_DATABASE_PATH = NOTMUCH_CONFIG_FIRST,
     NOTMUCH_CONFIG_MAIL_ROOT,