tty: Rework tty_driver structure

 - rename @t to @type and use protobuf constants here instead
 - for special features use @subtype just like kernel does
 - get rid of TTY_TYPE_ constants, we don't need them
 - drop @flags, we don't need it anymore

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>