Added "trim" command. Code maintenence.

Added "trim" op command to allow removal of players from players.txt who
haven't played any rounds of any game. Added checks on resetaway() and
resetsimple() to ensure only op users can use them. Changed "away",
"back" and "simple" commands to work without having to be joined in a
game. Modified "ping" command to not include players who have joined the
game.