Fix LDAP selection of DN after searching

Summary: Ref T1536. The "dn" property is unusual. Add code to find it.

Test Plan: Walked through this with @staticshock, who verified the fix.

Reviewers: staticshock, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6235