groonga-httpd-restart: re-add used variable...

Sorry...
groonga-httpd-restart: report an error on service isn't started
groonga-httpd-restart: remove needless wait

Because the PID file is already created.
groonga-httpd-restart: fix signal send timing

PID list of workers should be collected before worker stop signals.
All workers may be stopped between "kill" and "pgrep".