[krbdev.mit.edu #8579] duplicate caching of some cross-realm TGTs

[Greg Hudson via RT]

For client-driven cross-realm scenarios, I believe we should cache the 
TGTs we ask for, but not alternate TGTs.  If we cache alternate TGTs, we 
could have the same kind of scenario where we repeatedly cache an 
alternate TGT because the overall TGS operation fails.