psm(4): Add natural scrolling support to sysmouse protocol
This change enables natural scrolling with two finger scroll enabled
and when user is using a trackpad (mouse and trackpoint are not affected).
Depending on trackpad model it can be activated with setting of
hw.psm.synaptics.natural_scroll or hw.psm.elantech.natural_scroll sysctl
values to 1.
Evdev protocol is not affected by this change too. Tune userland client
e.g. libinput to enable natural scrolling in that case.
Submitted by: nyan_myuji.xyz
Reviewed by: wulf
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20447