Resolve LDAP registration DAO exception due to empty username

Summary:
When logging in as an LDAP user for the first time (thus registering), a DAO exception was being thrown because PhabricatorLDAPRegistrationController wasn't passing in a username to PhabricatorUser::setUsername().

Somewhat separately, since either the PHP LDAP extension's underlying library or Active Directory are returning attributes with lowercased key names, I have to search on sAMAccountName and look for the key samaccountname in the results; this is fine since the config allows these to be defined separately. However I found that PhabricatorLDAPProvider::retrieveUserName() was attempting to use the search attribute rather than the username attribute. This resolves.

Test Plan: Tested registration and login against our internal AD infrastructure; worked perfectly. Need help from someone with access to a functional non-AD LDAP implementation; I've added the original author and CCs from D2722 in case they can help test in this regard.

Reviewers: epriestley, voldern

Reviewed By: voldern

CC: voldern, aran, Korvin, auduny, svemir

Differential Revision: https://secure.phabricator.com/D3340