I modified `completion--complete-and-exit` so that the REQUIRE-MATCH
argument used with a function as value allows this function to exit with
'confirm in addition of nil/non-nil.

I then modified find-file-read-only so that it takes advantage of this:
- If you enter a wilcard you are now asked for confirm.
- If you enter a non existing filename you can't exit ([nomatch].

Note:
Of course the current behavior of find-file-read-only is fine, maybe a
little confusing but it's ok, this patch is just a proof of concept for
REQUIRE-MATCH used as a function with this new behavior.

The patches need to be reworked, didn't fully test the non interactive
usage and nothing is documented, particularly REQUIRE-MATCH in
completing-read, need also you commentaries because I guess not all the
corner cases are completed.

Thanks.

-- 
Thierry