upnp: Don't return loopback IPs in getOurIP.

[ghost]

Fixes #1620

net.LookupCNAME resolved to a loopback IP for me (127.0.1.1). The router can't port forward to this IP.

before

$ ./dcrd --upnp | grep -E "Successfully bound via UPnP|can't add UPnP port mapping"
-8: [WRN] SRVR: can't add UPnP port mapping: error 500 for AddPortMapping

after

$ git stash pop
$ go build
$ ./dcrd --upnp | grep -E "Successfully bound via UPnP|can't add UPnP port mapping"
-8: [WRN] SRVR: Successfully bound via UPnP to x.x.x.x:9108

[davecgh]

Thanks for the PR. This reads fine, but will need some testing before being merged to master.

[davecgh]

I did a bit of testing with this and verified the that behavior is more accurate than the existing code, so I'll approve this.

That said, it's still not accurate on systems with multiple addresses (e.g. both IPv4 and IPv6) nor those with multiple NICs. For example, the uPnP mapping fails completely on my Windows test system because it first tries to send the SSDP UDP from the address of one of the Hyper-V virtual NICs instead of the physical adapter in the host OS, then, if I hard code that in order to get past that, this getOurIP here returns a temporary IPv6 address instead of the IPv4 address it just tried to port map. Finally, the actual attempt to port map later fails because it tries to map the default port despite me having specified a different one in the listener.

[davecgh]

Also, I went ahead and fixed these things, but please note the code contribution guidelines as this didn't conform in several ways:

1. Neither the commit title nor the PR title followed the documented format
2. The commit message itself contains a GitHub issue number which is not allowed. Only the PR description may (and should) specify that.
3. The commit message itself doesn't contain any of the pertinent information as it's all on the GitHub PR.