kernel/sched: refactor _get_first_thread_to_unpend()

Modify _get_first_thread_to_unpend() so that it does not remove the
thread from the wait queue. Rename it to _find_first_thread_to_unpend()
to match the new behaviour.

This will be needed to fix a semaphore group bug.

Change-Id: I1b7531c3beecf3b6a86ecf88a93a02449edd0767
Signed-off-by: Benjamin Walsh <walsh.benj@gmail.com>