Add kmscon, a kms-based replacement for kernel VTs

Note that currently this depends on the default nixpkgs mesa and pango.
It may be possible to build more limited versions that don't e.g. depend
on the full X stack without limiting kmscon (which of course doesn't use
X).

Depends on libtsm, added in the same commit.

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