udevd: event - port spawn_wait() to sd-event

udevd: event - port spawn_wait() to sd-event

This allows us to drop the special sigterm handling in spawn_wait()
as this will now be passed directly to the worker event loop.

We now log failing spawend processes at 'warning' level, and timeouts
are in terms of CLOCK_BOOTTIME when available, otherwise the behavior
is unchanged.