Random order download

random order is generally better for the net. some people choose linear order to get the first and last chunks first, so they can preview avi's, for example.

_- but even with random_order_download enabled, first and last chunks still have a higher priority! Only in some patches, first and last lose their advantage when they're available from less than 10 sources, so that first sources don't spend their time sending those chunks again and again -- pango_