When delivering a signal with default disposition to the thread,
tdsigwakeup() increases the priority of the low-priority threads, to
give them a chance to be terminated timely. Also, kernel allows user
to signal kernel processes. The combined effect is that signalling
idle process bump a priority of the selected delivery thread, which
starts eating CPU.

Check for the delivery thread be an idle thread and do not raise its
priority then.

The signal delivery to the kernel threads must be opt-in feature.
Kernel thread should explicitely declare the ability to handle signals
directed to it. E.g., nfsd threads check for signal as an indication
of exit request.

Most threads do not handle signals at all, and queuing the signal to
them causes odd side-effects. Most innocent consequence is the memory
leak due to queued ksiginfo, which is never deleted from the sigqueue.
Code to prevent even queuing signals to the kernel threads is trivial,
but it requires careful examination of each call to kproc/kthread
creation to decide should the signalling be allowed. The commit is a
stop-gap measure which fixes the immediate case for now.

PR: 200493
Reported and tested by: trasz
Discussed with: trasz, emaste
Sponsored by: The FreeBSD Foundation
MFC after: 1 week