UPnP cleanup and fixes for #432

Code to fix the issues outlined in #432. The surgery was fairly invasive: I thought it best to leave the commits un-squashed so as to leave the transformations of the code as granular and obvious as possible. I'm happy to squash away if that's desired.

Notes:

* The generalization from 1 thing to n things made returning an error awkward. For example, if discovery fails on one device, that shouldn't affect the discovery of any other device. Mapping errors back to devices is awkward, requiring either parallel arrays or an additional field in the IGD struct, and much more complicated error-handling logic in the consumer. Since no error-handling logic beyond writing to log files was done by the upstream consumers, I elected to simply log errors to the debug log facilities instead of returning them to the consumer.
* I tested the changes with one of my local routers, Wireshark, and a miniupnpd instance in a VM. I didn't see a good way to automate testing of the discovery or port mapping mechanisms; the only readily testable portion of the API seems to be the search and port mapping request strings. These operations seem don't seem susceptible to regressions, but I can write tests if desired.
* Searching for multiple devices requires a bit of concurrency, which is frequently difficult to grok. I attempted to write the cleanest code possible and documented as well as I could, but I'd be particularly interested in feedback about how penetrable the solution is.
Ah, I see what you're getting at now, and I agree it is an issue. I see two options:

# Maintain a list of external sockets (IP address plus port) instead of an external port, and announce each external socket to the global discovery server. I'd have to read up on the discovery service to know how difficult that would be.
# Ignore all but the first discovered IGD and revert the logic in main.go to (essentially) its former state.

The code support multiple IGDs has certainly been more invasive than I expected, so I could see the later option being more attractive at this point. I'm happy to keep hacking away at the code if it seems useful, though. Thoughts?