constify ui_out_impl

This patch constifies ui_out_impl in struct ui_out, and various
instances of ui_out_impl.

This removes a couple of FIXME comments (near cli_ui_out_impl and
mi_ui_out_impl) that did not make sense to me.

Tested by rebuilding.

2014-02-28  Tom Tromey  <tromey@redhat.com>

	* cli-out.c (cli_ui_out_impl): Now const.  Remove comment.
	* cli-out.h (cli_ui_out_impl): Now const.
	* mi/mi-out.c (mi_ui_out_impl): Now const.  Remove comment.
	* ui-out.c (struct ui_out) <impl>: Now const.
	(default_ui_out_impl): Now const.
	(ui_out_new): Make 'impl' parameter const.
	* ui-out.h (ui_out_new): Update.