Make server interruptible

Use an interruptor to immediately wake up blocking calls on
server_stop().

app/src/server.h

@@ -10,6 +10,7 @@
 #include "adb.h"
 #include "coords.h"
 #include "options.h"
+#include "util/intr.h"
 #include "util/log.h"
 #include "util/net.h"
 #include "util/thread.h"
@@ -50,6 +51,8 @@ struct server {
     sc_cond cond_stopped;
     bool stopped;
 
+    struct sc_intr intr;
+
     sc_socket server_socket; // only used if !tunnel_forward
     sc_socket video_socket;
     sc_socket control_socket;