Change: Allow AI companies to start immediately.

Allow multiple AIs to possibly start in the same tick.
start_date = 0 becomes a special case, where random deviation does not occur.
If start_date was not already 0, then a minimum value of 1 must apply.

src/ai/ai.hpp

@@ -32,7 +32,7 @@ public:
 		START_NEXT_EASY   = DAYS_IN_YEAR * 2,
 		START_NEXT_MEDIUM = DAYS_IN_YEAR,
 		START_NEXT_HARD   = DAYS_IN_YEAR / 2,
-		START_NEXT_MIN    = 1,
+		START_NEXT_MIN    = 0,
 		START_NEXT_MAX    = 3600,
 		START_NEXT_DEVIATION = 60,
 	};