Add platforms for kmscon

Signed-off-by: Shea Levy <shea@shealevy.com>
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>
nixos: Split mesa setup from xserver.nix

With kmscon, it is now possible to have a system without X that still
needs the mesa setup in /run/opengl-driver

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

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