Switches from pcap_next() to pcap_next_ex() to better handle all error
conditions. This allows, for example, to have a non-zero exit code for
a Zeek process that fails to fully process all packets in a pcap file.
The change from #49 made it an error to not have a URI. That however
then led requests with an URI yet no version to abort as well.
Instead, we now check if the token following the method is an "HTTP/"
version identifier. If, so accept that the URI is empty (and trigger
a weird) but otherwise keep processing.
Adding test cases for both HTTP requests without URI and without
version.
