Add controls for players 6-8

[StormedBubbles]

lr-snes9x already recognizes when ports 1 and 2 are BOTH set to multitap, but only 5 players' worth of buttons are mapped. I added controls for players 6-8. This should resolve issue #257.

With these additions:

• I am able to play homebrew game "N-Warp Daisakusen" with 8 players
• Normal, single-port multitap usage seems unaffected

Please feel free to comment, change, or heckle any part of this. I am an amateur, so some of this code may be extraneous/redundant. 😃

[inactive123]

Thanks for this addition!