Switch team property from being a string to being an object

Switch team property from being a string to being an object

This should be cleaner as it avoids a bunch of bare string literals
all around the place that happen to match. Now, the player's team
property is a pointer into the game's array of teams. And for the case
when a player is not on a team, the game file maintains a "no_game"
sentinel object (with an empty-string name).