require a hash in fetch.php for external URLs FS#1769

Ignore-this: a66fc8874fb8e04b1258f2e71e35ed90

To avoid fetch.php being abused as anonymous forwarder or even proxy, now a
hash is needed for external ressources. This hash is automatically added by
the ml() function.

darcs-hash:20091006182001-7ad00-adf5f6275b0d7f76543f76d6196f1531b8c09e1c.gz