(svn r5999) -Feature: change the original date format to a 32 bits format based at the year 0.

The game date subsystem now allows someone to start in the year 0 and continue
up to the year 5 000 000. However, you currently cannot build anything before 
1920 as there is no newgrf support for dates before 1920 or after 2090 yet.

industry.h

@@ -28,7 +28,7 @@ struct Industry {
 	byte type;
 	byte owner;
 	byte color_map;
-	byte last_prod_year;
+	Year last_prod_year;
 	byte was_cargo_delivered;
 
 	uint16 index;