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

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