Adjust completion un/requoting in 'completing-read-multiple'

Adjust completion un/requoting in 'completing-read-multiple'

Teach 'completing-read-multiple' about 'completion--unquote', so it
can be used with completion tables that perform quoting.  Namely, this
allows reading multiple file names with:

  (completing-read-multiple "Files: " #'completion--file-name-table)

* lisp/emacs-lisp/crm.el (crm--table): New helper function.
(completing-read-multiple): Use it.