cli-script.c: Simplify using std::string, eliminate cleanups

cli-script.c: Simplify using std::string, eliminate cleanups

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

	* cli/cli-script.c (execute_control_command): Use std::string
	instead of cleanups.
	(locate_arg): Constify return type.
	(insert_args): Constify paremeter.  Simplify using std::string.
	Return a std::string.