Change VC-BACKEND-LOCKING-USER so that it doesn't call

Change VC-BACKEND-LOCKING-USER so that it doesn't call
VC-WORKFILE-MODIFIED?.  The problem is that the latter can call
VC-BACKEND-DIFF, which requires access to the CVS server.  However,
since VC-BACKEND-LOCKING-USER is called by VC-MODE-LINE, this meant
that every time the mode-line changed, we had to probe the server.
This patch eliminates that requirement.

The patch also tries to notice the results of user-initiated diffs,
and update the mode-line's idea of whether the file is modified.
However, Emacs doesn't seem to behave quite the same way.  Maybe this
is a bad idea and should be reverted.