sd-device: add API for triggering synthetic uevents with UUID

Since kernel 4.13 the kerne allows passing a UUID to generated uevents.
Optionally do so via a new sd_device_trigger_with_uuid() call, and add
sd_device_get_trigger_uuid() as helper to retrieve the UUID from a
uevent we receive.

This is useful for tracking uevents through the udev system, and waiting
for specific triggers.

(Note that the 4.13 patch allows passing arbitrary meta-info into the
uevent as well. This does not add an API for that, because I am not
convinced it makes sense — as it conflicts with our general rule that
events are "stateless" if you so will — and it complicates the interface
quite a bit).

This replaces #13881 in a way, which added a similar infra, but which
stalled, and whose synchronous settling APIs are somewhat problematic
and probably not material to merge.