#148 add/remove_terrain support

[Massimo Campostrini]

Currently, when a human player orders a unit to add/remove_terrain, if the unit has insufficient acp or material, the action fails silently (at least with the tcltk interface undel linux).

Ideally, a set of move/resupply/rest tasks should be set for the unit so that the action can be completed.

Less ideally, the player should be notified that the action is not performed and why.