Add support for retrieving the SNMPv2 agent address from a specific varbind

When forwarding SNMPv2 traps, the EMC Smarts Trap Forwarder will append the agent address in an extra snmpTrapAddress varbind (1.3.6.1.6.3.18.1.3.0).

In order to handle these and associate them with the correct node, we should enhance Trapd to support pulling the agent address from a specific varbind.