(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.

engine.h

@@ -76,8 +76,8 @@ typedef struct RoadVehicleInfo {
 typedef struct EngineInfo {
 	Date base_intro;
 	byte unk2;              ///< Carriages have the highest bit set in this one
-	byte lifelength;
-	byte base_life;
+	Year lifelength;
+	Year base_life;
 	byte railtype:4;
 	byte climates:4;
 	uint32 refit_mask;