(vc-buffer-backend): New function.

(vc-buffer-backend): New function.
Also new variable, local in all buffers.
(vc-kill-buffer-hook): Kill local vc-buffer-backend.

(vc-kill-buffer-hook): Don't put it on kill-buffer-hook.
(vc-file-clearprops): Function moved here.
(vc-workfile-version): Function moved here.
(vc-mode-line): Don't call vc-locking-user.

Add branch support for RCS;  treat CVS more like RCS and SCCS.
(vc-occurences, vc-trunk-p, vc-branch-p, vc-minor-revision)
(vc-branch-part): new functions that operate on RCS revision numbers.
(vc-status): Use the new property vc-workfile-version
and vc-locking-user (see vc.el).  Display "locking state" for CVS.
(vc-find-cvs-master): Search for file name case-sensitively, store
version number into the new property vc-workfile-version.
(vc-find-file-hook): kill any remaining properties.  Like this,
when re-finding a file (for example because it has changed on disk),
the version control state gets re-computed.
(vc-mode-line): CVS case: make the buffer read-only if the file
is unmodified.
(vc-kill-buffer-hook): Clear file's vc props when buffer is killed.