VC-Dir: Offer to register files before checking in

VC-Dir: Offer to register files before checking in

* lisp/vc/vc.el (vc-only-files-state-and-model): Rewrite
checking that all files are in compatible VC states.  In
particular, consistently return 'edited' when all files are
either added, removed or edited, instead of allowing the return
value to depend on the order of the files in VC-Dir, and offer
to registered unregistered files if doing so would allow the
operation to proceed.
(vc-compatible-state): Delete.
(vc-next-action): Replace call to vc-compatible-state.
Document, in this command's docstring, the new feature
implemented in vc-only-files-state-and-model.
* etc/NEWS: Announce the new feature.

(cherry picked from commit 253364e2c71bfaaa3ca623ed23f1cae6548a16c7)