MFC r200447,201703,201709-201710:

MFC r200447,201703,201709-201710:
In current code, threads performing an interruptible sleep
will leave the waiters flag on forcing the owner to do a wakeup even
when the waiter queue is empty.
That operation may lead to a deadlock in the case of doing a fake wakeup
on the "preferred" queue while the other queue has real waiters on it,
because nobody is going to wakeup the 2nd queue waiters and they will
sleep indefinitively.
A similar bug, is present, for lockmgr in the case the waiters are
sleeping with LK_SLEEPFAIL on.

Add a sleepqueue interface which does report the actual number of waiters
on a specified queue of a waitchannel and track if at least one sleepfail
waiter is present or not. In presence of this or empty "preferred" queue,
wakeup both waiters queues.

Discussed with:	kib
Tested by:	Pete French <petefrench at ticketswitch dot com>,
		Justin Head <justin at encarnate dot com>

git-svn-id: svn://svn.freebsd.org/base/stable/8@202576 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f