I looked through the NSPR socket creation functions that InitWithAddress
uses to see which errors they could return, and placed appropriate comments
in ErrorAccordingToNSPR.
The test coverage is not great; in particular, I wasn't able to find a way
to elicit "address in use" errors from Windows (although I could from
Linux); the web says that Windows is much more relaxed about binding
listening sockets than Unix derivatives. I'm interested in suggestions.
