JoyMega joypad support

Please add support for emulating a MegaDrive/Genesis joypad connected through a JoyMega adapter.

Currently, FireHawk-HDD, the Vampire Killer patch, the JoyMega, and the JoyTest tools do support this adapter. It's a pretty simple adapter and it will probably not be hard to emulate on openMSX. The code is probably readily available on MegaDrive emulators.

It could be used like this:

plug joyporta joymega6button (connects a 6-button MegaDrive joypad to the MSX)

plug joyporta joymega3button (connects a 3-button MegaDrive joypad to the MSX)

For more info on the JoyMega adapter, see here:
http://frs.badcoffee.info/hardware/joymega-en.html

PS: The 3-button and 6-button MegaDrive joypads have, in fact, 4 and 8 buttons, respectively.