Make bufdaemon and bufspacedaemon use kthread_suspend_check instead of
kproc_suspend_check. In r329612 bufspacedaemon was turned into a thread
of the bufdaemon process causing both to call kproc_suspend_check with the
same proc argument and that function contains the following while loop:

while (SIGISMEMBER(p->p_siglist, SIGSTOP)) {
wakeup(&p->p_siglist);
msleep(&p->p_siglist, &p->p_mtx, PPAUSE, "kpsusp", 0);
}

So one thread wakes up the other and the other wakes up the first again,
locking up UP machines on shutdown.

Also register the shutdown handlers with SHUTDOWN_PRI_LAST + 100 so they
run after the syncer has shutdown, because the syncer can cause a
situation where bufdaemon help is needed to proceed.

PR: 227404
Reviewed by: kib
Tested by: cy, rmacklem