udevd: kill hanging event processes after 30 seconds

Some broken kernel drivers load firmware synchronously in the module init
path and block modprobe until the firmware request is fulfilled.

The modprobe-generated firmware request is a direct child device of the
device which caused modprobe to run. Child device event are blocked until
the parent device is handled. This dead-locks until the kernel firmware
loading timeout of 60 seconds is reached.

The hanging modprobe event should now time-out and allow the firmware
event to run before the 60 second kernel timeout.