Support (spy) actions against the actor's own tile.

Pressing 'd' will pop up the spy action selection dialog with target(s) from
the tile the acting unit currently is located on.
Bug fix: can_actor_unit_move(): Fix same tile detection.

The test that the unit isn't already located at the target tile compared a
tile id to a tile. This made it unable to detect that the action unit
already was on the target tile. Compare tile to tile in stead.