Implement functionality to actually start the session

Password checking is already implemented using PAM.

Still need to implement functionality to actually start the session.

Possible approaches:

* Use xdm but change out its greeter with our LoginWindow code. License seems compatible. https://gitlab.freedesktop.org/xorg/app/xdm
* Write something from scratch so that we can keep this under the BSD-2-Clause license
* Find existing cli tools that we can use to do the job and just call them (some combination of `login` and `xsession`?)

Fallbacks
* Port code from slim
* Port code from pcdm https://github.com/helloSystem/hello/issues/254