ldap: Privilege separation for LDAP attribute retrieval

Allow simpleSAMLphp to use a different LDAP user for retrieving
the users attributes.

Patch by Victoriano Giralt <victoriano@uma.es>.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1679 44740490-163a-0410-bde0-09ae8108e29a