sd-device-monitor: expose low-level functions

sd-device-monitor: expose low-level functions

To make it work without sd-event.

Prompted by recent chat:
> Hey all!
> reading man libudev, it says to use sd-device instead now. I've read that
> APIs header file and it seems it no longer has an equivalent to libudev's
> udev_monitor_get_fd, which AFAICT means I have to use sd-event to watch
> for events I'm interested in. I know I can "embed" sd-event in other event
> loops I might already have, but that seems overkill when I'm only interested
> in this one type of event and don't need sd-event for anything else.