install: add new installer implementation

install: add new installer implementation

This new installer will replace the current code of "systemctl enable"
but also be available via D-Bus. It adds a couple of new features:

- Mask/Unmask calls
- Reenable call
- Preset call
- Support for enabling units temporarily (i.e. in /run/systemd instead
  of /etc/systemd)
- Enumeration of installed units
- Support for out-of-search-path units

systemctl and D-Bus are not hooked up with this yet