MFC r306355, r306474, r306746:

r306355:
Add Elantech trackpad support

Elantech trackpads are found in some laptops like the Asus UX31E. They
are "synaptics compatible" but use a slightly different protocol.

Elantech hardware support is not enabled by default and just like
Synaptic or TrackPoint devices it should be enabled by setting
tunable, in this case hw.psm.elantech_support, to non-zero value

PR: 205690
Submitted by: Vladimir Kondratyev <wulf@cicgroup.ru>
MFC after: 1 week

r306474:
Replace explicit TUNABLE_INT to sysctl with CTLFLAG_TUN

• Replace tunables-only hw.psm.synaptics_support, hw.psm.trackpoint_support, and hw.psm.elantech_support with respective sysctls declared with CTLFLAG_TUN. It simplifies checking them in userland, also makes them easier to get discovered by user
• Get rid of debug.psm.loglevel and hw.psm.tap_enabled TUNABLE_INT declaration by adding CTLFLAG_TUN to read/write sysctls that were already declared for these tunables.

Suggested by: jhb

r306746:
Fix extended buttons support on synaptic clickpad

Fix regression introduced by r306355 on synaptic clickpads with
extended buttons (buttons stopped working)

PR: 205690
Submitted by: Vladimir Kondratyev <wulf@cicgroup.ru>