net: enable runtime-integrated network pollster on netbsd

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13080043
net: fix garbage connection close in dual stack tests

This may possibly be the root cause of flaky dual stack tests.

Update issue 4176
Update issue 5001

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13050043