start using the RLL RELEASE procedures

So far, we immediately disable the RF channel without following a proper
RLL RELEASE procedure.  This patch changes this.

If we locally terminate the connection, the channel allocator now triggers a
RLL RELEASE REQuest, which is responsed by the MS with a RLL RELEASE CONFirm,
based on which we send the RF CHANnel RELease to the BTS.

If the MS terminates the connection, we receive a RLL RELEASE INDication,
based on which we trigger RF CHANnel RELease to the BTS.
diff --git a/openbsc/src/chan_alloc.c b/openbsc/src/chan_alloc.c
index e3d6ae6..a66f70e 100644
--- a/openbsc/src/chan_alloc.c
+++ b/openbsc/src/chan_alloc.c
@@ -239,7 +239,7 @@
 	}
 
 	DEBUGP(DRLL, "Recycling the channel with: %d (%x)\n", lchan->nr, lchan->nr);
-	rsl_chan_release(lchan);
+	rsl_release_request(lchan, 0);
 	return 1;
 }