[krbdev.mit.edu #2580] need realm identification at service level

[Ken Raeburn via RT]

For some uncommon(?) configurations like running a single service on a machine in one 
realm, while other services on the machine are in a different realm, we should have a 
mechanism for identifying the realm of a single service, perhaps as an extension to the 
domain-realm mapping in the config file.

Doing this with KDC-based referrals shouldn't be hard, when we get there, but I suspect in 
the majority of cases it'll be done for testing or short-lived services for which the 
administrative hassle or delay in updating the (production) KDC with the new service makes it 
a poor choice; that would presumably be the case for adding referral data too.  In such cases, 
though, we really don't need a solution that scales up well, and tweaking the config file is 
probably adequate.