introduce names for user backends - IUserBackend

apps/user_ldap/user_ldap.php

apps/user_ldap/user_proxy.php

apps/user_webdavauth/user_webdavauth.php

lib/private/user/database.php

lib/private/user/dummy.php

lib/private/user/http.php

lib/private/user/manager.php

lib/private/user/user.php

lib/public/iuserbackend.php

tests/lib/user/manager.php

tests/lib/user/user.php