Statefull reset and unblock BVCs and sending flow control messages

The flow control interval can be set via VTY.
diff --git a/src/gprs_rlcmac.h b/src/gprs_rlcmac.h
index a5ac775..e7a68a4 100644
--- a/src/gprs_rlcmac.h
+++ b/src/gprs_rlcmac.h
@@ -62,6 +62,7 @@
 };
 
 struct gprs_rlcmac_bts {
+	uint8_t fc_interval;
 	uint8_t cs1;
 	uint8_t cs2;
 	uint8_t cs3;