Active multiplexing is a nice feature as it allows several pointing devices
(such as touchpad and external mouse) use their native protocols at the
same time. Unfortunately many manufacturers do not implement the feature
properly even though they advertise it. The problematic implementations are
never fixed, since Windows by default does not use this mode, and move from
one BIOS/model of laptop to another. When active multiplexing is broken
turning it on usually results in touchpad, keyboard, or both unresponsive.

With PS/2 usage on decline (most of PS/2 devices in use nowadays are
internal laptop touchpads), I expect number of users who have laptops with
working MUX implementation, docking stations with external PS/2 ports, and
who are still using external PS/2 mice, to be rather small. Let's flip the
default to be OFF and allow activating it through i8042.nomux=0 kernel
option.  We'll also keep DMI table where we can record known good models.

Acked-by: Jiri Kosina <jkosina@suse.cz>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>