abstract transport to static driver functions and opaque driver specific information.

This splits a transport into static driver specific functions for the common
iscsi commands. Optionally, a driver specific opaque memory is introduced
which is currently only used by iSER transport.
Last a lot of functions changed to static.

Signed-off-by: Peter Lieven <pl@kamp.de>

lib/task_mgmt.c

@@ -83,7 +83,7 @@ iscsi_task_mgmt_async(struct iscsi_context *iscsi,
 
 	if (iscsi_queue_pdu(iscsi, pdu) != 0) {
 		iscsi_set_error(iscsi, "failed to queue iscsi taskmgmt pdu");
-		iscsi->t->free_pdu(iscsi, pdu);
+		iscsi->drv->free_pdu(iscsi, pdu);
 		return -1;
 	}