[krbdev.mit.edu #7766] Keyring last_change_time implementation isn't useful

[Greg Hudson via RT]

The cc_keyring.c implementation of last_change_time from r20743 records 
the last change time in memory only, so all you find out is the last time 
that particular handle was resolved, initialized, or stored into.  You 
don't find out anything about the underlying cache.

The kernel key API does not appear to have timestamps (we should verify 
this), so fixing this will probably require adding a new key to the cache 
keyring.