[infiniband] Create a general management agent

Generalise the subnet management agent into a general management agent
capable of sending and responding to MADs, including support for
retransmissions as necessary.

src/include/gpxe/ib_mad.h

@@ -201,6 +201,8 @@ struct ib_smp_class_specific {
  *****************************************************************************
  */
 
+#define IB_SA_CLASS_VERSION			2
+
 struct ib_rmpp_hdr {
 	uint32_t raw[3];
 } __attribute__ (( packed ));