libretro2joy : A libretro core that emulates USB gamepads with uinput.

This might be handy for remapping buttons on physical gamepads, or synchronizing
gamepad data over the network using RetroArch's netplay code.

Currently, it maps the libretro "gamepad" to the layout of Logitech RumblePad 2.

TODO:
 * Launch a given executable with load_game_special.
   - This will make synchronizing native games easier when using netplay.
   - Distant future: optionally hook SDL calls and put the A/V into libretro.
 * Visual feedback of some sort for the emulator window.
   - Draw a controller for each one connected, perhaps.
 * Optional (#ifdef?  somehow chosen at runtime?) Xbox-style button mappings.
   - Haven't looked into a Windows port yet, but it might be an easier target.
   - Games these days tend to assume Xbox button layout.