kernel - cyapa driver operational in PS/2 mode

* For now don't try to emulate the event infrastructure or trackpad
  infrastructure.

* Requires ig4 for chrome.  Include this in /boot/loader.conf for now:

    ig4_load="YES"
    cyapa_load="YES"

* Implement a character device interface that is compatible with PS/2.
  The X configuation would thus be something like this:

    Section "InputDevice"
	    Identifier  "Mouse0"
	    Driver      "mouse"
	    Option      "Protocol" "ps/2"
	    Option      "Device" "/dev/cyapa0-67"
    EndSection

* Implement simple left/middle/right button emulation depending on where
  on the trackpad you push the button.

* Do not implement simple-touch for buttons, that creates all sorts of
  false positives.  The trackpad has an actual button (the whole pad) if
  you push hard enough.
build - Document usb4bsd build requirements

* Document them in the obvious place
kernel - remove i2c debugging for ig4

* Remove a kprintf("I") in the interrupt path for ig4.
Sync umass driver with FreeBSD
Re-add the cam rescan yet again. I shouldn't delete this every time I merge.
build - Add dev/smbus to kernel module build

* Add dev/smbus to the kernel module build, it required some Makefile
  adjustments in platform/pc*
Document usb4bsd build instructions for 32 bit users, too.