Autodiscovery returns something useful when OC is not in DC

* Honor dcerpc_mapiproxy:samdb_url parameter when available
   * And use machine account credentials when loading Samba environment in OCS manager
* Fix autodiscover URLs when the OpenChange is behind a proxy
   * Check first HTTP_X_FORWARDED_HOST header instead of HTTP_X_FORWARDED_SERVER
as OpenChange may be proxied and the client is unable to this server directly.