add support for tests to check systemd unit properties and dependencies of systemd units

[djhaynes]

A number of Linux based distributions have switched their init system from SysVinit to systemd. The systemd init system is different from the old SysVinit style in that it does not have a concept of a runlevel, but instead has targets and various forms of service activation. For example, it can start a service based on achieving a target (e.g. graphical), time of day, connection to a socket, or a number of other ways. Please see http://www.freedesktop.org/wiki/Software/systemd/ for more information.

Because of this, the old runlevel test just does not work because the configuration information is not kept in /etc/rc.d/ nor do the kind of start up methods fit in the concept of the runlevel test. Therefore, we would like to propose a new test and a set of objects related to it for OVAL 5.11 inclusion. We do not believe the old runlevel test should be deprecated as some systems still use it.

Please see the following oval-developer-list post for additional information.

http://making-security-measurable.1364806.n2.nabble.com/Proposal-for-OVAL-5-11-systemd-test-tp7583274.html