ps2: add support for mice with extra/side buttons

This enables the ps2 controller to process mouse events for buttons 4 and 5.
Additionally, distinct definitions for the ps2 mouse button state are
introduced. The legacy definitions from console.h are not used anymore.

Signed-off-by: Fabian Lesniak <fabian@lesniak-it.de>
Message-id: 20161206190007.7539-3-fabian@lesniak-it.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>