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_config.hpp

@@ -34,6 +34,7 @@ public:
 
 	/* virtual */ int GetSetting(const char *name) const;
 	/* virtual */ void SetSetting(const char *name, int value);
+	/* virtual */ void AddRandomDeviation();
 
 	/**
 	 * When ever the AI Scanner is reloaded, all infos become invalid. This