[krbdev.mit.edu #6006] APIs for service principal selection

[Alexandra Ellwood via RT]

Client applications need to be able to select the appropriate service principal name to acquire 
credentials for.  Clients may be operating in an environment using referrals, cross-realm or 
without DNS reverse resolution.

This parent bug should be used to track all requests of this nature so we can come up with 
more sophisticated versions of the existing APIs (eg: krb5_sname_to_princ).