Input: mousedev - remap BTN_FORWARD from BTN_LEFT to BTN_MIDDLE

In mousedev the BTN_LEFT and BTN_FORWARD were mapped to mouse button 0,
causing that the user space program cannot distinguish between them through
/dev/input/mice. All mice have BTN_LEFT, but not all have BTN_MIDDLE (e.g.
Clevo D410J laptop). Mapping BTN_FORWARD to mouse button 2 makes the
BTN_FORWARD button useful on this laptop.

Signed-off-by: Marton Nemeth <nm127@freemail.hu>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>