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.
s/had/hand/g

Basically what I mean is that it would be nice to get UPnP code up to the state where it is able to provide a bunch of IP:PORT pairs, and then the current code which uses the UPnP module would just pick a random port for `externalPort` variable out of the bunch of pairs the UPnP code returns.

This way the UPnP part is done, and we only need to change the announcement code.

As for the `connection status`, I guess any code using the UPnP module externally will not care about the `connection status`, what they will care about is the currently active IGDs, so I think the `connection status` should only be used by the UPnP code internally to work out which pairs to return when someone asks for them.