x/net/ipv6: re-enable IPV6_PATHMTU for TCP on darwin

Now this option works fine on latest darwin kernels.

Please be informed that using mixed RFC 2292 and RFC 3542 options
on darwin easily becomes a root cause of various kernel crashes.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/166500043

ipv6/sys_darwin.go

@@ -67,6 +67,9 @@ func init() {
 		sockOpts[ssoReceivePathMTU].level = iana.ProtocolIPv6
 		sockOpts[ssoReceivePathMTU].name = sysIPV6_RECVPATHMTU
 		sockOpts[ssoReceivePathMTU].typ = ssoTypeInt
+		sockOpts[ssoPathMTU].level = iana.ProtocolIPv6
+		sockOpts[ssoPathMTU].name = sysIPV6_PATHMTU
+		sockOpts[ssoPathMTU].typ = ssoTypeMTUInfo
 	}
 }