Two finger scrolling

[nia <nia%NetBSD.org@localhost>]

I've just committed a handful of changes to synaptics.c that give me
smooth two-finger scrolling out of the box.

I'm curious how many others are using multitouch touchpads and whether
you notice any improvement.

The existing approximation (sysctl hw.synaptics.finger_scroll...) wasn't
working for me at all, tweaking hw.synaptics.finger_scroll-min either
gave me no scrolling or registered normal mouse movements as scrolling

I suspect my fingers are significantly smaller than whoever wrote this
code. Was anyone using this successfully, and how does scrolling compare
after my changes?