We are not able to remove custom rules: remove the empty stubs (which
originate from the original OpenDPI code).
`ndpi_guess_protocol_id()` is only called on the first packet of the
flow, so the bitmask `flow->excluded_protocol_bitmask` is always empty,
since we didn't call any dissectors yet.
Move another hash function to the dedicated source file.
Some notes:
* libinjection: according to https://github.com/libinjection/libinjection/issues/44,
it seems NULL characters are valid in the input string;
* RTP: `rtp_get_stream_type()` is called only for RTP packets; if you
want to tell RTP from RTCP you should use `is_rtp_or_rtcp()`;
* TLS: unnecessary check; we already make the same check just above, at
the beginning of the `while` loop
