x/net/ipv4: enable IP_STRIPHDR by default on darwin

According to github.com/opensource-apple/xnu.git, IP_STRIPHDR sticky
socket option was introduced in xnu-517 (Darwin 7.0, OS X 10.3).

LGTM=iant
R=iant, gobot
CC=golang-codereviews
https://golang.org/cl/176480043
1 file changed