kernel: Allow pending w/o wait_q for scheduler API cleanup

The mailbox code was written to use the _remove_thread_from_ready_q()
API directly, which would be good to get out of the scheduler internal
API.  What it really wanted to do is to mark a thread "PENDING"
without actually adding it to a wait queue, which is sane enough (the
message stores the "thread to wake up on receipt" handle).

So allow that naturally in the _pend_thread() API by passing a NULL
wait_q.  Really a wait_q needn't be the only way a thread can block.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>