Change return type of find_and_open_script

Change return type of find_and_open_script

This changes find_and_open_script to return a
gdb::optional<open_script>, where open_script is a new type
encapsulating the two return values.  The new type helps avoid
cleanups in the callers.

ChangeLog
2017-08-03  Tom Tromey  <tom@tromey.com>

	* cli/cli-cmds.c (find_and_open_script): Change return type.
	Remove "streamp" and "full_path" parameters.
	(source_script_with_search): Update.
	* auto-load.c (source_script_file): Update.
	* cli/cli-cmds.h (find_and_open_script): Change type.
	(open_script): New struct.