Input: implement input filters

Sometimes it is desirable to suppress certain events from reaching
input handlers and thus user space. One such example is Mac mouse
button emulation code which catches certain key presses and converts
them into button clicks as if they were emitted by a virtual mouse.
The original key press events should be completely suppressed,
otherwise user space will be confused, and while keyboard driver
does it on its own evdev is blissfully unaware of this arrangement.

This patch adds notion of 'filter' to the standard input handlers,
which may flag event as filtered thus preventing it from reaching
other input handlers. Filters don't (nor will they ever) have a
notion of priority relative to each other, input core will run all
of them first and any one of them may mark event as filtered.

This patch is inspired by similar patch by Matthew Garret but the
implementation and intended usage are quite different.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>