--list-protocol was printing a *:// entry, which looked strange at best.
The "*" protocol was used to always match everything, so stream_cb.c
could hook in custom protocols with a prefix chosen by the API user.
Change it instead so that an empty protocol list means "match all",
which also gets rid of the special-cased "*" entry.
seek_fn is supposed to return the new file offset, or a negative error
code. Our code doesn't use the offset, and only wants to know if any
errors happened. The int cast is completely broken and might treat a
successful seek as failed depending on whether the sign bit will be set.
