HTTP/1.0 does not mandate a Content-Length header in responses since keep-alive connections aren't a thing; the connection closure then signals the end of the response.
This change requires the URL metadata line for processing HTTP/1.0 data, so it plays well with `warc-dump-responses --meta`.

%n takes a signed int so is incompatible with size_t; further, sscanf could in theory return a negative value (EOF).