net: don't set wrong option for controlling tcp keepalive on openbsd
net: don't set wrong option for controlling tcp keepalive on openbsd

Fixes issue 8679.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/141730043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go