MFC r346455-r346458, r348520, r348529, r348817, r348818

r346455:
psm(4): Add support for 4 and 5 finger touches in synaptics driver

While 4-th and 5-th finger positions are not exported through PS/2
interface, total number of touches is reported by MT trackpads.

r346456:
psm(4): do not process gestures when palm is present

Ignoring of gesture processing when the palm is detected helps to reduce
some of the erratic pointer behavior.

This fixes regression introduced in r317814

Reported by: Ben LeMasurier <ben@crypt.ly>

r346457:
psm(4): respect tap_disabled configuration with enabled Extended support

This fixes a bug where, even when hw.psm.tap_enabled=0, touchpad taps
were processed.
tap_enabled has three states: unconfigured, disabled, and enabled (-1, 0, 1).
To respect PR kern/139272, taps are ignored only when explicity disabled.

Submitted by: Ben LeMasurier <ben@crypt.ly> (initial version)

r346458:
psm(4): give names to synaptics commands

Submitted by: Ben LeMasurier <ben@crypt.ly>

r348520:
psm(4): Add Elantech touchpad IC type 15 found on Thinkpad L480 laptops

PR: 238291
Submitted by: Andrey Kosachenko <andrey.kosachenko@gmail.com>

r348529:
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
Differential Revision: https://reviews.freebsd.org/D20447

r348817:
psm(4): Fix Elantech trackpoint support.

Sign bits for X and Y motion data were taken from wrong places.

PR: 238291
Reported by: Andrey Kosachenko <andrey.kosachenko@gmail.com>
Tested by: Andrey Kosachenko <andrey.kosachenko@gmail.com>

r348818:
psm(4): Add extra sanity checks to Elantech trackpoint packet parser.

Add strict checks for unused bit states in Elantech trackpoint packet
parser to filter out spurious events produces by some hardware which
are detected as trackpoint packets. See comment on r328191 for example.

Tested by: Andrey Kosachenko <andrey.kosachenko@gmail.com>