Make it work over tcpip

"adb reverse" currently does not work over tcpip (i.e. on a device
connected by "adb connect"):
<https://issuetracker.google.com/issues/37066218>

To work around the problem, if the call to "adb reverse" fails, then
fallback to "adb forward", and reverse the client/server roles.

Keep the "adb reverse" mode as the default because it does not involve
connection retries: when using "adb forward", the client must try to
connect successively until the server listens.

Due to the tunnel, every connect() will succeed, so the client must
attempt to read() to detect a connection failure. For this purpose, when
using the "adb forward" mode, the server initially writes a dummy byte,
read by the client.

Fixes <https://github.com/Genymobile/scrcpy/issues/5>.

app/src/net.h

@@ -21,6 +21,7 @@
 SDL_bool net_init(void);
 void net_cleanup(void);
 
+socket_t net_connect(Uint32 addr, Uint16 port);
 socket_t net_listen(Uint32 addr, Uint16 port, int backlog);
 socket_t net_accept(socket_t server_socket);