Use ldap_search() to find the user's DN in sub OUs of base DN

Create new `search_sub_ous` setting and admin field for it. If set, when authenticating search sub-OUs of the base DN for uid=username.

We need this because our LDAP directory uses OUs and sub-OUs to organize user accounts. We want to allow login from any account within ou=People,dc=example,dc=com but a typical DN is like uid=jschmoe,ou=Staff,out=People,dc=example,dc=com.