add setting to xcvs-taginfo to prevent deleting tags entirely

as a first (trivial) step towards http://drupal.org/node/90483, the attached patch adds a new setting to the xcvs-* scripts so repo admins can prevent users from deleting tags entirely.

#90483 is better, since it'll let folks clean up their own mess until they create a release node, but this would be an acceptable stop-gap measure to use when we first deploy, if #90483 doesnt' get finished...