(svn r5887) -Cleanup: move date related functions, defines and variables to date.[ch]

-Cleanup: fix whitespace related coding style issues in date.[ch]
-Cleanup: make original comments doxygen compatible and remove/change outdated comments

engine.c

@@ -16,6 +16,7 @@
 #include "variables.h"
 #include "train.h"
 #include "newgrf_cargo.h"
+#include "date.h"
 
 EngineInfo _engine_info[TOTAL_NUM_ENGINES];
 RailVehicleInfo _rail_vehicle_info[NUM_TRAIN_ENGINES];