Remove most cleanups from linux-thread-db.c

Remove most cleanups from linux-thread-db.c

This removes most (but not all) cleanups from linux-thread-db.c.
std::string and std::vector are used in place of manual memory
management.

The remaining cleanup in linux-thread-db.c uses
make_cleanup_free_char_ptr_vec, which requires a somewhat bigger
change.

Regression tested by the buildbot.

ChangeLog
2018-02-24  Tom Tromey  <tom@tromey.com>

	* linux-thread-db.c (try_thread_db_load_from_pdir_1)
	(try_thread_db_load_from_dir, thread_db_load_search): Use
	std::string.
	(info_auto_load_libthread_db_compare): Return bool.  Change
	argument types.
	(info_auto_load_libthread_db): Use std::vector, std::string.
	Remove cleanups.