[9fans] scroll mouse support

[Russ Cox <russcox@gmail.com>]

Thanks to Andrey, the kernel code on sources (you'll
need to compile a new binary) has support for 
mice with scroll wheels.  Rio and acme scroll their
text windows in response to the scroll wheel.

Jim and I decided that autodetection is too hard and
not likely to work anyway (I've never seen XFree86 get
it right), so you have to tell the kernel when you have
a scroll mouse (aka intellimouse) by setting mouseport=ps2intellimouse
in plan9.ini, or just by doing echo intellimouse >/dev/mousectl
at some point later.

Most Unix/Windows programs scroll half or full screens
per click, and making things worse, different scroll mice
send clicks at different rates.  Rob and I played with this
for a while a few months ago and settled on the following.
By default, rio and acme scroll one line per click, which
feels a lot smoother than big jumps.  If you want some
other behavior, you can set $mousescrollsize in your
environment to, say, 5 (5 lines) or to 25% (quarter screen).

Russ