identity: Give players unique unspoofable player IDs.

Players generate private keys (stored in the .sta files). Replies to NET_PING are now signed, so if the ping of a
player is not ∞, and you recognise the player ID of the player, then you know the ping is from the same player as
before.
identity: Show players in different colours, depending on whether the player was seen before or is lagging.

Also, don't start the game if a player is lagging, since in that case, the game might anyway not actually start
until the player finally times out.