libsystemd: add sd-device library

libsystemd: add sd-device library

This provides equivalent functionality to libudev-device, but in the
systemd style. The public API only caters to creating sd_device objects
from for devices that already exist in /sys, there is no support for
listening for monitoring events or creating devices received over
the udev netlink protocol.

The private API contains the necessary functionality to make sd-device
a drop-in replacement for libudev-device, but which we would not
otherwise want to export.