Add sd_notify() parameter to change watchdog_usec during runtime.
Application can change watchdog_usec value by
sd_notify like this. Example. sd_notify(0, "WATCHDOG_USEC=20000000").
To reset watchdog_usec as configured value in service file,
restart service.
Notice.
sd_event is not currently supported. If application uses
sd_event_set_watchdog, or sd_watchdog_enabled, do not use
"WATCHDOG_USEC" option through sd_notify.
