Introduce ui_file_as_string

Introduce ui_file_as_string

ui_file_as_string is a variant of ui_file_xstrdup that returns a
std::string instead of a xmalloc'ed char *.  The idea is using the new
function to eliminate "make_cleanup (xfree, ...)"  cleanups
throughout.

Following patches will make use of this.

gdb/ChangeLog:
2016-11-08  Pedro Alves  <palves@redhat.com>

	* ui-file.c (do_ui_file_as_string, ui_file_as_string): New
	functions.
	* ui-file.h: Include <string>.
	(ui_file_as_string): New declaration.