Remove GOOS_solaris ifdef from netpoll code,
instead introduce runtime edge/level triggered IO flag.
Replace armread/armwrite with a single arm(mode) function,
that's how all other interfaces look like and these functions
will need to do roughly the same thing anyway.

LGTM=rsc
R=golang-codereviews, dave, rsc
CC=golang-codereviews
https://golang.org/cl/55500044