(vc-directory): Kill existing vc-dired buffers for this directory.

(vc-directory): Kill existing vc-dired buffers for this directory.
Provide a better header.  Corrected the check whether any files were
found at all (don't display a listing in this case).  Under CVS,
display cvs-status rather than vc-locking-user.
(vc-next-action-on-file): When doing a check-in in vc-dired-mode, find
the file in another window.
(vc-next-action-dired): Update dired listing while processing the
files.
(vc-next-action): Check whether a check-in comment is really needed
for this mass operation.
(vc-checkout): Resynch the buffer, even if it's not current.
(vc-dired-state-info, vc-dired-update-line): New functions.
(vc-dired-prefix-map): Added local definition for `g' and `='.
(vc-dired-reformat-line): Simplified.  Erase the hardlink count from
the listing, because it doesn't relate to version control.
(vc-rcs-release, vc-cvs-release, vc-sccs-release): New variables, may
be set by the user.
(vc-backend-release, vc-release-greater-or-equal, vc-backend-release-p):
New Functions.
(vc-do-command): Allow FILE to be nil.
(vc-backend-checkin): When creating a branch, don't bother to unlock
the old version if this is RCS 5.6.2 or higher.
(vc-next-action-on-file): Allow lock-stealing only if RCS 5.6.2 or
higher.
(vc-backend-admin, vc-backend-checkin): If available, use ci -i and -j.
Updated Developer's Notes.