Handle migration from pre-1.7 databases with master key kvno != 1

Handle migration from pre-1.7 databases with master key kvno != 1

krb5_dbe_lookup_mkvno assumes an mkvno of 1 for entries with no
explicit tl_data.  We've seen at least one pre-1.7 KDB with a master
kvno of 0, violating this assumption.  Fix this as follows:

* krb5_dbe_lookup_mkvno outputs 0 instead of 1 if no tl_data exists.
* A new function krb5_dbe_get_mkvno translates this 0 value to the
  minimum version number in the mkey_list.  (krb5_dbe_lookup_mkvno
  cannot do this as it doesn't take the mkey_list as a parameter.)
* Call sites to krb5_dbe_lookup_mkvno are converted to
  krb5_dbe_get_mkvno, except for an LDAP case where it is acceptable
  to store 0 if the mkvno is unknown.

ticket: 6650
target_version: 1.7.1
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@23676 dc483132-0cff-0310-8789-dd5450dbe970