improvements to some emacs search tagging functions

[Jameson Graef Rollins]

This series fixes a couple of minor annoyances/bugs with tagging in
search-mode:

* The old search-tag-thread function would not tag regions, and the
  main search-tag function could not just take whole tagging
  operations as input.  This series fixes that so that all tagging
  operations, by thread or by region, are now handled with the main
  search-tag function.  Tags can be specified as a sting or a list, or
  the user can be prompted (with tab completion) if no tag is
  specifically specified.

* Tag tab completion was often incomplete or would throw errors.  Tag
  tab completion is now more full-featured.

The last patch, which fix tag tab completion for excluded tags, needs
Mark Walters new exclude fixes [0] to work.

The series removes one public-facing function
(notmuch-search-tag-thread), and renames another
(notmuch-search-archive-thread --> notmuch-search-archive).  But I
think it's worth it since it makes for a much cleaner interface.

jamie.

[0] id:"1333234442-28616-1-git-send-email-markwalters1009 at gmail.com"