Implement keyboard/mouse control

To control the device from the computer:
 - retrieve mouse and keyboard SDL events;
 - convert them to Android events;
 - serialize them;
 - send them on the same socket used by the video stream (but in the
 opposite direction);
 - deserialize the events on the Android side;
 - inject them using the InputManager.

app/src/strutil.h

@@ -11,4 +11,4 @@ size_t xstrncpy(char *dest, const char *src, size_t n);
 // join tokens by sep into dst
 // returns the number of chars actually written (max n-1) if no trucation
 // occurred, or n if truncated
-size_t xstrjoin(char *dst, const char *const tokens[], char sep, size_t);
+size_t xstrjoin(char *dst, const char *const tokens[], char sep, size_t n);