(vc-diff): If file is unchanged, ask for the version

(vc-diff): If file is unchanged, ask for the version
number to compare with.
(vc-retrieve-snapshot): If no NAME is specified, check out
latest versions of all unlocked files.
(vc-next-action-on-file): For CVS files with implicit checkout: if
unmodified, don't do anything.
(vc-clear-headers): Regexp more restricted, so as not to destroy file
contents by mistake.
(vc-backend-merge-news): Better analysis of status reported by CVS.
Set file properties accordingly.