linux-thread-db.c: use bool where possible in thread_db code

linux-thread-db.c: use bool where possible in thread_db code

I happened to be looking at this code and noticed we could replace ints
by bools at a few places.

gdb/ChangeLog:

	* linux-thread-db.c (try_thread_db_load_1): Change return type
	to bool.
	(try_thread_db_load): Likewise.
	(try_thread_db_load_from_pdir_1): Likewise.
	(try_thread_db_load_from_pdir): Likewise.
	(try_thread_db_load_from_sdir): Likewise.
	(try_thread_db_load_from_dir): Likewise.
	(thread_db_load_search): Likewise.
	(has_libpthread): Likewise.
	(thread_db_load): Likewise.