Add a taskqueue_quiesce(9) KPI.

This is similar to taskqueue_drain_all(9) but will wait for the queue
to become idle before returning instead of only waiting for
already-enqueued tasks to finish. This will be used in the opensolaris
compat layer.

PR: 227784
Reviewed by: cem
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17975