introduce ui_file_write_for_put

introduce ui_file_write_for_put

This introduces a small helper function, ui_file_write_for_put.  It is
a wrapper for ui_write that is suitable for passing directly to
ui_file_put.

This patch also updates one existing spot to use this new function.

gdb/ChangeLog
2014-12-12  Tom Tromey  <tromey@redhat.com>

	* ui-file.h (ui_file_write_for_put): Declare.
	* ui-file.c (ui_file_write_for_put): New function.
	* mi/mi-out.c (do_write): Remove.
	(mi_out_put): Use ui_file_write_for_put.