Remove listen mode

Changing behaviour so that instead of rebinding after a successful
migration and continuing as an ordinary server, we simply quit with a
0 exit code and let our caller restart us as a server if they want to.
This means that everything in listen.c, listen.h, and anything making
reference to a rebind address is unneeded.

src/control.h

@@ -1,10 +1,14 @@
 #ifndef CONTROL_H
 #define CONTROL_H
 
+/* We need this to avoid a complaint about struct server * in
+ * void accept_control_connection
+ */
+struct server;
 
 #include "parse.h"
 #include "mirror.h"
-#include "control.h"
+#include "serve.h"
 #include "flexnbd.h"
 #include "mbox.h"