Avoid null pointer mess from undefined units.
Added Binding ability for Giant Spiders.
Bind effect causes a unit to lose Flying.
Added special accessor for flying.

Effects can cause a unit to lose flying (e.g., Bound).  Centralize this
check all in one place.  Also, Bound units can't trample.
New unit Dendroid, new ability Steadfast.

Steadfast units can retaliate unlimited times.