Merge the two `Commentary' sections.

Merge the two `Commentary' sections.
(locate-ls-subdir-switches): New user option.
(locate): Update for other changes.
(locate-mode-map): Restore Dired binding for mouse-2.
Bind `locate-mouse-view-file' to M-mouse-2.
Bind `l' to `locate-do-redisplay'.
(locate-main-listing-line-p, locate-do-redisplay): New functions.
(locate-mouse-view-file, locate-tags, locate-find-directory):
Print message if used outside main listing.
(locate-mode): Update docstring.  Make `*Locate*' buffer read-only.
Various changes to support inserted subdirectories.
(locate-insert-header): Change header of *Locate* buffer.