[DON'T MERGE] [Reviewer: Andy] HSS interop fixes

Andy,

Please can you review these HSS interop fixes?  They include

*   setting the identity and realm correctly when running on IPv6 (using a domain name if we have one - not just hard-coding to the ip6.arpa domain)
*   advertising support for Cx/3GPP rather than just Cx
*   specifying the correct vendor ID and name
*   (adding the 3GPP2 vendor, which we already advertise support for, although it turns out we don't need it yet).

I've live-tested and all the UTs pass.

Matt