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

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