[krbdev.mit.edu #8776] Replay Cache FD Leak

So far I haven't been able to find a leak in the replay cache code, and 
I can't find records of how previous reports of this kind of issue were 
resolved.

Note that each GSS acceptor credential handle (if it contains a krb5 
credential) holds a replay cache handle, which holds an open file 
descriptor.  So if the application is leaking GSS credential handles, 
it would manifest as an fd leak in the process.