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

raboof/net,chenzlabs/net,flowlo/go-net,cmars/tools,golang/net,flowlo/go-net,w4ngyi/net,kiip/net,subuk/net,DramaFever/golang.org-x-net,dafyddcrosby/net,mckinney-subgraph/net,cesanta/goxnet,muzining/net,mckinney-subgraph/net,yangxianjun/net,raboof/net,muzining/net,cmars/tools,veliamsli/net,gomini/net,kiip/net,subuk/net,cmars/tools,foreversmart/net,zabawaba99/net,dafyddcrosby/net,albertjin/golang-net,cmars/tools,w4ngyi/net,foreversmart/net,veliamsli/net,albertjin/golang-net,yangxianjun/net,gomini/net,DramaFever/golang.org-x-net,cesanta/goxnet,zabawaba99/net,chenzlabs/net