Add KDC helpers for current key and kvno

Add KDC helpers for current key and kvno

Add a simple static inline function current_kvno() to safely fetch the
current kvno of a principal entry, and use it where we currently write
entry->key_data[0].key_data_kvno.

Add a function get_first_current_key() to find and decrypt the first
valid current key from an entry.  Use it in get_local_tgt() and when
selecting a ticket encryption key during AS and TGS processing.

Add a local_tgt_key field to krb5_kdcpreauth_rock_st and use it in
add_freshness_token() so we don't have to decrypt it again.