Add a test to the end of agentx_varbind_finalize(), after completing the
full OID, that checks if we're >= searchrange.end. If so, just make it
an endOfMIBView.

OK tb@

According to RFC2741 section 6.1.1 an agentx-response-pdu shouldn't have
the NON_DEFAULT_CONTEXT set. Remove the argument from ax_response().

OK tb@

Make sure that unpacking of an AgentX OID doesn't contain more than 128
elements.

OK tb@