New command icomplete-force-complete-and-exit

New command icomplete-force-complete-and-exit

* lisp/icomplete.el (icomplete-force-complete-and-exit):
New command
(http://lists.gnu.org/archive/html/emacs-devel/2015-05/msg00461.html)
(http://lists.gnu.org/archive/html/emacs-devel/2015-05/msg00516.html).
(icomplete-minibuffer-map): Bind C-j to it.
(icomplete-forward-completions, icomplete-backward-completions):
Mention the new command in the docstring.

* lisp/minibuffer.el (minibuffer-force-complete-and-exit): Revert
the previous fix for bug#17545.