Add SNMP datagram parsing support.

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.

scripts/base/protocols/snmp/main.bro

@@ -0,0 +1,15 @@
+##! Enables analysis of SNMP datagrams.
+
+module SNMP;
+
+export {
+}
+
+const ports = { 161/udp, 162/udp };
+
+redef likely_server_ports += { ports };
+
+event bro_init() &priority=5
+	{
+	Analyzer::register_for_ports(Analyzer::ANALYZER_SNMP, ports);
+	}