Changes to support filenames as tags too and provided

Changes to support filenames as tags too and provided
a drop-in replacement for list-tags.
(find-tag-noselect): Recognize filenames as valid tags too.
(find-tag-file-order): New variable holds function to check for match
for a file name used as a tag.
(last-tag-file): New variable; stores the filename looked for via
find-tag family of functions.
(find-tag-in-order): If the tag is a file name, position at file beg.
(etags-recognize-tags-table): Added new var find-tag-file-order to
tags-table-format variables.  Added tag-filename-match-p to the
list for find-tag-tag-order.
(tag-filename-match-p): New function.
(list-tags): Rewritten for speed.
(tags-list-functions-in-file): New subroutine for list-tags.
(tags-locate-file-in-tags-table): New function locates a
file in `tags-table-list'.