Add module to use kmscon instead of linux-console for VTs

This required some changes to systemd unit handling:

* Add an option to specify that a unit is just a symlink
* Allow specified units to overwrite systemd-provided ones
* Have gettys.target require autovt@1.service instead of getty@1.service

Signed-off-by: Shea Levy <shea@shealevy.com>