Allow non-matching input, but require confirmation

[Stefan Monnier]

Currently, the `require-match' argument allows three different behaviors:
1) nil = allow any non-matching input.
2) t   = only allow matching input.  Automatically complete
         without confirmation.
3) neither nil nor t =  only allow matching input, but require confirmation
         after automatic completion.

I'd like to add a fourth one:

4) confirm-only = allow any non-matching input but require confirmation
                  if the input is not a valid completion.

Also I propose to use it for reading the file name in `find-file'.


        Stefan