[krbdev.mit.edu #7685] kadmind caches master key activation times

[Greg Hudson via RT]

The master key activation times list is a tl-data value on the K/M entry 
containing {kvno, time} tuples, and determines which master key is 
considered "active" for the purpose of encrypting new keys.

kadmind reads this list at startup by calling 
krb5_dbe_fetch_act_key_list, and stores it in a global variable 
active_mkey_list.  This value is never updated during the lifetime of 
the kadmind process.  As a result, kdb5_util use_mkey operations will 
not affect a running kadmind process.  This appears to have been 
considered in the project page, but is not documented in the kdb5_util 
man page.

We can either document this or fix it.  Fixing it means looking up the 
K/M DB entry for every key change operation 
(cprinc/chpass/chrand/setkey), which is probably not a big deal.  We 
already do that for the history key for chpass, which is the most common 
key change operation, and if we ever implement #1221 we could collapse 
those into one lookup (for new databases).