changes to vc-update

[Dan Nicolaescu]

I just checked in a change to vc-update to make it resolve conflicts in
case `merge-news' returned true. (Now doing C-x v + for a file under CVS
that has conflicts will turn on smerge-mode).
 
vc-cvs-merge-news seem to have had a bug for ever: in case of a conflict 
  (vc-cvs-command nil 0 file "update")
threw an error (because of the "0" arg), so the output of the command
was never parsed. So I changed "0" to nil.n

Some other commands in vc-cvs.el might have the same issue.

IMO these changes are OK, but it be nice if someone else would look over
them.

Thanks

        --dan