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

libexec/rc/rc.d/auditd

@@ -26,7 +26,9 @@ auditd_stop()
 {
 
 	/usr/sbin/audit -t
-	sleep 1
+	if [ -n "$rc_pid" ]; then
+		wait_for_pids $rc_pid
+	fi
 }
 
 load_rc_config $name