userdbd: modernize signal handling

userdbd: modernize signal handling

Let's make use of SD_EVENT_SIGNAL_PROCMASK so that we don't have to mask
the signals manually. Let's use sd_event_set_exit_signal() instead of
rolling our own SIGTERM/SIGINT handling. Let's use "floating" event
sources instead of keeping references on our own.

Let's also debug log if we can't enable watchdog handling.