udev: restore udevadm settle timeout

udev: restore udevadm settle timeout

Commit 9ea28c55a2 (udev: remove seqnum API and all assumptions about
seqnums) introduced a regresion, ignoring the timeout option when
waiting until the event queue is empty.

Previously, if the udev event queue was not empty when the timeout was
expired, udevadm settle was returning with exit code 1.  To check if the
queue is empty, you could invoke udevadm settle with timeout=0. This
patch restores the previous behavior.

(David: fixed timeout==0 handling and dropped redundant assignment)