alq, siftr: add panic/debugger checks to shutdown hooks

Don't try to gracefully terminate the pkt_manager thread if the
scheduler is not running.

We should not attempt to shutdown ald if RB_NOSYNC is set, and must not
if the scheduler is stopped (the function calls wakeup()).

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42340

(cherry picked from commit d79a9edb5ce162c1ba49e12e5c93b894e6a25ad2)