Add proper support for confirm-nonexistent-file-or-buffer

Add proper support for confirm-nonexistent-file-or-buffer
values `confirm' and `confirm-after-completion'.
(ido-read-internal, ido-buffer-internal, ido-file-internal):
Setup and handle require-match arg according to c-n-f-o-b.
(ido-show-confirm-message): New dynamic variable.
(ido-exit-minibuffer): Request confirmation for non-existing file
or buffer according to confirm-nonexistent-file-or-buffer.
(ido-decorations): Add 11th element for "confirm" message.
(ido-completions): Show confirmation message when requested.