New backend functions `delete-file' and `repository-hostname'.

New backend functions `delete-file' and `repository-hostname'.
(vc-stay-local): New var.  Mostly taken from vc-cvs-stay-local.
(vc-stay-local-p): New fun.  Adapted from vc-cvs-stay-local-p.
(vc-diff-switches-list): Revert to the Emacs-21.[123] semantics.
Mark as obsolete.
(vc-delete-file): New command.
(vc-default-rename-file): New function.
(vc-rename-file): Use it.
Be careful to disallow renaming if the file is locked or out-of-date.
(vc-ensure-vc-buffer, vc-next-action-on-file, vc-insert-headers)
(vc-cancel-version, vc-annotate): Use buffer-file-name variable.