surface: Make bullets hit the ground.
surface: Make Tank to shoot enemies and shot Tank takes damage.

Also revived death effects for Tank.
surface: Add customizable max health value to Tank init script.
surface: Make Tank steer towards enemy if not controlled by a Player.

The sample mission demo7.nut now demonstrates this by constantly supplying
Tanks if ones get killed.
The demonstation seems to occasionally raise exceptions regarding
std::lists.
It should have something to do with self deletion of Tanks in
takedamage().
surface: Increased vertical accuracy of Tank's main gun.

We don't need to consider gravity when aiming with tank gun, because the
target object is likely on the ground.  We won't shoot at a target in the
air.

Also increased demonstration area and Tank count in demo7.nut.
Tank's enemy sense range is increased to make it fun.
surface: Make SurfaceEntity's race neutral; no one will try to attack it.
Colored neutral Entity with light gray in GLWentlist.