Integrate PCP and NAT-PMP

[rawdigits]

I can't find evidence this is in wide use, but worth supporting these, as it may help folks behind NAT.

[SlyngDK]

I have created a PR on this, you can try testing it https://github.com/slackhq/nebula/pull/579

I am running a test setup with it and works fine with miniupnpd on the gateway.

Config needed:

punchy:
  port_mappings.pcp:
    enabled: true

[comex]

For what it's worth, my Xfinity-provided home router:

• Has a 'port restricted cone' NAT, i.e. sending a UDP packet to a given IP and port only allows responses from that specific IP and port. Thus UDP hole punching is not possible.
• Does not support PCP or NAT-PMP, as far as I can tell.
• Does support UPnP IGD.

So there do exist common setups where hole punching does not work but automated port forwarding does work (though not specifically PCP or NAT-PMP). I can make do with manual port forwarding, but it would be nice if Nebula 'just worked' in this situation.