[Reviewer: Mike] Support any realm that the HSS gives us

Mike,

Please can you review the following fix (which fixes #465).  It's slightly more complicated than I suggested in my pull request for the PJSIP change because I now need to check that the realm provided by the user matches that in the digest (previously PJSIP did this for us implicitly, because there was only one possible value).

I've live-tested, and the UTs all run cleanly (although I have one line of code coverage I need to add).

Thanks,

Matt