Add interface for handling systemd unit files.

This is used twice (now): once for the init_t and once for udev_t.
systemd comes with the feature, that service files might link
to /dev/null.  For this feature another rule (null_device_t) is
needed here.
Merge branch 'bugfix/756729-udev-use-systemd-unit-files' into experimental
Updated changelog: udev must use systemd unit files