Dump through control socket

Note this removes SIGUSR1, SIGUSR2, and the graph dumping config option.
It seems cleaner to do everything through the control socket.

src/control_common.h

@@ -26,6 +26,11 @@ enum request_type {
 	REQ_STOP,
 	REQ_RELOAD,
 	REQ_RESTART,
+	REQ_DUMP_NODES,
+	REQ_DUMP_EDGES,
+	REQ_DUMP_SUBNETS,
+	REQ_DUMP_CONNECTIONS,
+	REQ_DUMP_GRAPH,
 };
 
 #define TINC_CTL_VERSION_CURRENT 0