Put an optional IdempotentCallback in bufchains.

The callback has the same semantics as for packet queues: it triggers
automatically when data is added to a bufchain, not when it's removed.

cmdgen.c

@@ -126,6 +126,10 @@ char *x_get_default(const char *key)
 void sk_cleanup(void)
 {
 }
+void queue_idempotent_callback(IdempotentCallback *ic)
+{
+    assert(0);
+}
 
 void showversion(void)
 {