Patch by Martin.Lorentzson@telia.com.

Patch by Martin.Lorentzson@telia.com.
(vc-cvs-sticky-date-format-string): New variable.
(vc-cvs-sticky-tag-display): New variable.
(vc-cvs-mode-line-string): Add sticky-tag to the mode-line.
(vc-cvs-checkin): If the input revision is a valid symbolic tag
name, we create it as a branch, commit and switch to it.
(vc-cvs-retrieve-snapshot): Set file-property sticky-tag.
(vc-cvs-valid-symbolic-tag-name-p): New function.
(vc-cvs-parse-sticky-tag): New function.
(vc-cvs-parse-entry): Added parsing of sticky tags.