auditd_stop: wait_for_pids instead of sleeping

It's faster and more reliable to wait_for_pids than to sleep 1.

cem@ suggested just to remove auditd_stop() and use the rc.subr default
stop action (SIGTERM instead of audit -t), which has a built-in
wait_for_pids. That may be a better solution.

Discussed with: cem
Reviewed by: asomers
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23223