This augments the existing notmuch_message_get_filename by allowing
the caller access to all filenames in the case of multiple files for a
single message.
To support this, we split the iterator (notmuch_filenames_t) away from
the list storage (notmuch_filename_list_t) where previously these were
a single object (notmuch_filenames_t). Then, whenever the user asks
for a file or filename, the message object lazily creates a complete
notmuch_filename_list_t and then:
For notmuch_message_get_filename, returns the first filename
in the list.
For notmuch_message_get_filenames, creates and returns a new
iterator for the filename list.
The new implementation is simply a talloc-based list of strings. The
former support (a list of database terms with a common prefix) is
implemented by simply pre-iterating over the terms and populating the
list. This should provide no performance disadvantage as callers of
thigns like notmuch_directory_get_child_files are very likely to
always iterate over all filenames anyway.
This new implementation of notmuch_filenames_t is in preparation for
adding API to query all of the filenames for a single message.
