Instead of killing the 'watchdog' subshell and leaving a sleep for

rcshutdown_timeout (normally 30s) around re-parented to init, make
sure both go away using pkill -P.

While noone normally notices this for the system shutdown, it helps for
cleanly shutting down trusted jails.
Found without a killall in the base system, which in rc.d/jail normally
ensures that all processes of a jail to be stopped will be killed.

Reviewed by:	silence on current@
MFC after:	4 weeks

etc/rc.shutdown

@@ -98,7 +98,7 @@ done
 # Terminate the background watchdog timer (if it is running)
 #
 if [ -n "$_rcshutdown_watchdog" ]; then
-	kill -TERM $_rcshutdown_watchdog >/dev/null 2>&1
+	pkill -TERM -P $_rcshutdown_watchdog >/dev/null 2>&1
 fi
 
 # Insert other shutdown procedures here