Support user_specific source in kolab_users_directory (Bifrost#T236416)

Move kolab_auth/kolab_auth_ldap to libkolab/lib/kolab_ldap.
It ends up much simpler to add user_specific support and unify some
code than replace use of kolab_auth_ldap with rcube_ldap.

This means that libkolab plugin does not depend on kolab_auth plugin
anymore, but kolab_auth depends on libkolab, which is better situation.