gpsdate.init: Use GPSDATE_{HOST,PORT} from /etc/default/gpsdate

so far only the sytemd service file used those variables, but not the
init script.  Fix that.

gpsdate.init

@@ -45,7 +45,7 @@ case "$1" in
         # Unconditionally start gpsdate daemon because we want to run it even
         # if wd_keepalive wasn't running
         echo "Starting gpsdate..."
-        $DAEMON $gpsdate_options
+        $DAEMON ${GPSDATE_HOST} ${GPSDATE_PORT} $gpsdate_options
     fi
     ;;