Support searching for users to find their LDAP entry

Summary:
 - the current LDAP auth flow expects a DN to look like
   cn=ossareh,ou=Users,dc=example,dc=com
 - however many LDAP setups have their dn look something like
   cn=Mike Ossareh,ou=Users,dc=example,dc=com

Test Plan:
 Test if logins work with a LDAP setup which has cn=Full Name
 instead of cn=username.

 To test you should ensure you set the properties needed to
 trigger the search before login as detailed in conf/default.conf.php

Reviewers: epriestley

CC: mbeck, aran, Korvin

Differential Revision: https://secure.phabricator.com/D3072

conf/default.conf.php

@@ -642,6 +642,14 @@ return array(
   // The attribute to be regarded as 'username'. Has to be unique
   'ldap.search_attribute' => '',
 
+  // Perform a search to find a user
+  // Many LDAP installations do not have the username in the dn, if this is
+  // true for you set this to true and configure the username_attribute below
+  'ldap.search-first'         => false,
+
+  // The attribute to search for if you have to search for a user
+  'ldap.username_attribute' => '',
+
   // The attribute(s) to be regarded as 'real name'.
   // If more then one attribute is supplied the values of the attributes in
   // the array will be joined