Add a sniffer

The examples are module/sniffer/tls and module/sniffer/http. The module value implements spec.SnifferModule (settings, Info, and NewSniffer(), which prepares anything needed once), and the value it builds implements spec.Sniffer. Sniff is called from many connections at once:

• return a spec.SniffResult when the bytes are recognised;
• return spec.ErrSniffNeedMoreData while they are only a prefix of your protocol;
• return any other error as soon as they cannot be your protocol, so a connection is not held waiting;
• only trim what you read; do not lowercase or otherwise rewrite it.

Register it with moduleRegistry.Sniffers().Register(<name>.NewSnifferModule).