[Freeciv-Dev] DoS attack server with allowconnect (PR#1139)

[Vasco Alexandre Da Silva Costa <vasc@xxxxxxxxxxxxxx>]

I've been thinking a bit, and IMHO the best way to stop this from
happening is to always make new players able to join in pregame, and
to make it always possible to join to pre-existing human players.

Since we have no way to know if a player is thrustworthy or not, we cannot
allow someone to completely block entry to a game for everyone.

The scenario of someone switching everyone to AI, disabling connections to
AI players and logging out is still possible. However the game will end
eventually.

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Vasco Alexandre da Silva Costa @ Instituto Superior Tecnico, Lisboa