Gamemode improvements

[rahlzel]

File: https://github.com/jaggerestep/ColonialMarines/blob/master/code/game/gamemodes/xeno/xeno.dm

Potential reference from another source: https://github.com/ParadiseSS13/Paradise/blob/master/code/game/gamemodes/xenos/xenos.dm

1. Set required_players to 3 and increase minimum Xeno spawns at round-start from 1 to AT LEAST 2. Thread: http://newedenstation.com/showthread.php?tid=562
2. The gamemode is not using the required_players variable. Changing this does nothing. If only one player hits "Ready", the game will still start.
3. The gamemode also seems to be ignoring player choices for Xeno candidacy in the Character Setup screen, and will randomly select players without taking the candidacy into consideration.
4. Same problem as above, but with forcing LO/Commander on a player who often doesn't know how to play it or would rather log out than play it. Suggestion: Create a candidacy for "Be LO/Commander", set it to "Yes" by default, and allow "No" to only force these players as a last resort.
5. Win conditions: Marines that snuck onto the Nostromo while Aliens wiped through the Sulaco will prevent the round from ending. The Aliens should win if there aren't any alive/infected Marines on the Sulaco, with a message like "The Marines have died or fled the station!" Also, living HIT members still at Centcom will prevent an alien win. Furthermore, Marines that are huggered but not yet dead will prevent a win. An alien win should probably happen when all Marines are at least infected. This will prevent "where is the last human??" stalemates that last for a very long time.