rc.subr: Use pwait in wait_for_pids.

This waits for the requested process(es) to terminate, rather than polling
with an interval of 2 seconds.

If pwait is not available, the old method is used.

PR:		conf/132766
Reviewed by:	dougb

etc/rc.subr

@@ -390,7 +390,7 @@ wait_for_pids()
 		_list=$_nlist
 		echo -n ${_prefix:-"Waiting for PIDS: "}$_list
 		_prefix=", "
-		sleep 2
+		pwait $_list 2>/dev/null || sleep 2
 	done
 	if [ -n "$_prefix" ]; then
 		echo "."