uterm: vt: add fake-vt logic for debugging

uterm: vt: add fake-vt logic for debugging

This adds a very limited non-multi-seat-capable fake-VT logic. If you use
this in production it will break your multi-seat systems so do not use it
except for debugging.

On SIGUSR1 we activate fake VTs and on SIGUSR2 we deactivate them. The
signals must be sent from a priviledged process. Kernel signals are
ignored.

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