Previously, struct string was used here. However,
bittorrent_fetch_callback does not initialize response.magic,
and parse_bittorrent_tracker_response changes response->source
to point to data that must not be freed. So the util/string.h
functions are not actually safe to use on these objects.
For this reason, it is safer to use a separate type.
... mainly bittorrent:// and bittorrent://x
The BitTorrent URL is supposed to contain an embedded URL pointing to a
metainfo file. If this is not the case a "custom" error message will be
shown. Also fixes calling of free_list() on an uninitialized list.
Closes bug 729.
