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

ipv4/sys_darwin.go

@@ -29,6 +29,7 @@ var (
 		ssoReceiveDst:         {sysIP_RECVDSTADDR, ssoTypeInt},
 		ssoReceiveInterface:   {sysIP_RECVIF, ssoTypeInt},
 		ssoHeaderPrepend:      {sysIP_HDRINCL, ssoTypeInt},
+		ssoStripHeader:        {sysIP_STRIPHDR, ssoTypeInt},
 		ssoJoinGroup:          {sysIP_ADD_MEMBERSHIP, ssoTypeIPMreq},
 		ssoLeaveGroup:         {sysIP_DROP_MEMBERSHIP, ssoTypeIPMreq},
 	}
@@ -58,8 +59,6 @@ func init() {
 		ctlOpts[ctlPacketInfo].parse = parsePacketInfo
 		sockOpts[ssoPacketInfo].name = sysIP_RECVPKTINFO
 		sockOpts[ssoPacketInfo].typ = ssoTypeInt
-		sockOpts[ssoStripHeader].name = sysIP_STRIPHDR
-		sockOpts[ssoStripHeader].typ = ssoTypeInt
 		sockOpts[ssoMulticastInterface].typ = ssoTypeIPMreqn
 		sockOpts[ssoJoinGroup].name = sysMCAST_JOIN_GROUP
 		sockOpts[ssoJoinGroup].typ = ssoTypeGroupReq