Avoid abbreviation, preferring notmuch_config_get_maildir_synchronize_flags

Since the name of the configuration parameter here is:

	maildir.synchronize_flags

the convention is that the functions to get and set this parameter
should match it in name. Hence:

       notmuch_config_get_maildir_synchronize_flags

etc. (as opposed to notmuch_config_get_maildir_sync).