tty: Introduce tty types

Instead of calling case() with majors all over the places lets
introduce own enum for tty types and use it instead.

Because we're using not @major numbers now but taking @minors
into account as well, this brings more strict check of which
kind of terminals we can dump now thus it's potentially should
fix the cases when we're trying to c/r terminals which we don't
understand yet (in particular /dev/console [5:1]).

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>