Remove usage of find_thread when calling resume_status_pending_p

Remove usage of find_thread when calling resume_status_pending_p

Replace with find_thread.  Instead of setting the flag in the callback,
make the callback return true/false, and check the result against NULL
in the caller.

gdb/gdbserver/ChangeLog:

	* linux-low.c (resume_status_pending_p): Return bool, remove
	flag_p argument.
	(linux_resume): Use find_thread.