Freeciv

Sort enum values in terrain_enums.def by their identifier wherever possible and reasonable, and mark them with the style option from #688.

• terrain_class: happens to already be in order, unlikely to have new values added ~> might as well mark it as sorted
• terrain_alteration: order not relevant (only affects rulesave output); sort it
• terrain_flag_id: User flags are currently not autogenerated (because of TER_LAST), but they happen to be at the end anyway ~> sort it
• mapgen_terrain_property: order not relevant; sort it