Implement new Linux table systemd_units, listing systemd units

[rcipkins]

This PR implements a table for systemd_units.

+------------------+---------+------------+--------------+-----------+
| name             | type    | load_state | active_State | sub_state |
+------------------+---------+------------+--------------+-----------+
| example1.service | serivce | loaded     | active       | running   |
+------------------+---------+------------+--------------+-----------+
| example2.mount   | mount   | loaded     | inactive     | mounted   |
+------------------+---------+------------+--------------+-----------+

Requires: #6562