Correctly activate master keys in pre-1.7 KDBs

Correctly activate master keys in pre-1.7 KDBs

Starting with 1.7, databases are created with actkvno tl-data in the
K/M entry which gives the initial master key version an activation
time of 0.  A database created before 1.7 will not have this tl-data,
but we should behave in the same way as we do for a more recent
database.

Move the actkvno list synthesis code from krb5_dbe_fetch_act_key_list
to krb5_dbe_lookup_actkvno so it applies to kdb5_util commands as well
as libkadm5.  Synthesize the same list as we would have initialized
the KDB with, with an activation time of 0 for the earliest master
key.

ticket: 7686
target_version: 1.12
tags: pullup