This supports parsing of SNMPv1 (RFC 1157), SNMPv2 (RFC 1901/3416), and
SNMPv2 (RFC 3412). An event is raised for each SNMP PDU type, though
there's not currently any event handlers for them and not a default
snmp.log either. However, simple presence of SNMP is currently visible
now in conn.log service field and known_services.log.
