(vc-next-action-on-file): Changed doc string, CVS case.

(vc-next-action-on-file): Changed doc string, CVS case.
(vc-backend-checkin): Clear master properties before doing the
check-in.  In the CVS case, catch error and display an appropriate
message in the echo area.
(vc-backend-revert): Clear master properties, because some might get
outdated by the operation.
(vc-backend-merge-news): Scan the output of "cvs update" and find out
whether conflicts have occurred.
(vc-finish-logentry): Delete the *VC-log* buffer *before* doing the
actual check-in. That leads to a better display in case the check-in
fails.
(vc-latest-on-branch-p): Defined it for CVS in the obvious way.
(vc-next-action-on-file): Removed special CVS cases. At this level,
CVS is now mostly handled like the other backends.
(vc-backend-checkout): CVS case: Clear sticky tag as default when
doing a verbose checkout. (This used to be done in
vc-next-action-on-file.)