net: implement netbsd sockoptip
net: implement netbsd sockoptip

Provide sockoptip for NetBSD, based on sockoptip for OpenBSD.

R=golang-dev, rsc, mikioh.mikioh, minux.ma
CC=golang-dev
http://codereview.appspot.com/6308053
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go