uvt: new library implementing VTs in user-space

uvt: new library implementing VTs in user-space

UVT is based heavily on the old cdev-sessions. It uses CUSE/FUSE to
implement virtual terminals in user-space.

This move into a library allows to use it in other projects, too. There is
no reason to limit it to kmscon sessions. In fact, we will remove the
cdev-sessions, soon and make kmscon a stand-alone terminal emulator
without any session capability.
Instead, the uvtd program will provide the VT emulation.

This library is not finished, nor ready for use. However, feel free to
contribute patches so we can eventually release a stable API.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>