This means that any user attempt to remove a rear engine will tell the user to move the front engine instead
This fixes the assert when moving multiheaded engines (introduced in r3144)
Note: to make old savegames use this feature, some engines might be turned around in order to link engines in pairs
-Codechange: train subtype is now a bitmask
This allows fast access to info like if it is a wagon or engine and if it is in front and so on
Note: savegame version bump
