Add initial multithreading support and example

This is the basic support for doing i/o in a separate worker thread.
It is still not threads safe but a start.
Now we need to protect all variables such as outqueue, waitpdu
and friends.

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>

lib/pdu.c

@@ -940,7 +940,17 @@ iscsi_timeout_scan(struct iscsi_context *iscsi)
 int
 iscsi_queue_pdu(struct iscsi_context *iscsi, struct iscsi_pdu *pdu)
 {
-	return iscsi->drv->queue_pdu(iscsi, pdu);
+        int ret;
+
+#ifdef HAVE_MULTITHREADING
+        iscsi_mt_mutex_lock(&iscsi->iscsi_mutex);
+#endif /* HAVE_MULTITHREADING */
+	ret = iscsi->drv->queue_pdu(iscsi, pdu);
+#ifdef HAVE_MULTITHREADING
+        iscsi_mt_mutex_unlock(&iscsi->iscsi_mutex);
+#endif /* HAVE_MULTITHREADING */
+
+        return ret;
 }
 
 void