fix krb5_ldap_iterate to handle NULL match_expr and open_db_and_mkey to use KRB5_KDB_...

fix krb5_ldap_iterate to handle NULL match_expr and open_db_and_mkey to use KRB5_KDB_SRV_TYPE_ADMIN

When I ran kdb5_util dump I had two initial problems.  First, the LDAP
plugin was not finding the bind DN because open_db_and_mkey() was
passing KRB5_KDB_SRV_TYPE_OTHER to krb5_db_open().  When I change this
to KRB5_KDB_SRV_TYPE_ADMIN then the ldap_kadmind_dn parameter is used
from krb5.conf and a valid bind DN is found.  Second,
krb5_ldap_iterate() will core dump when it is called withy a NULL
match_expr arg.  This is how dump_db calls krb5_db_iterate().  I updated
krb5_ldap_iterate() to use a default_match_expr of "*" if match_expr ==
NULL.

ticket: new

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@18736 dc483132-0cff-0310-8789-dd5450dbe970