Change auto_load_objfile_script_1 to use std::string

Change auto_load_objfile_script_1 to use std::string

This replaces some manual string manipulation in
auto_load_objfile_script_1 with std::string, simplifying the code and
allowing the removal of some cleanups.

Tested by the buildbot.

2018-03-17  Tom Tromey  <tom@tromey.com>

	* auto-load.c (auto_load_objfile_script_1): Use std::string.