More Xconq bug fixes

[Hans Ronne]

My latest bunch of bug fixes has been checked in. Most of them are
concerned with how the kernel and AI code deal with non-mobile units that
can fire. This kind of unit is not very common in existing games, and was
therefore not handled correctly.

One new feature is the ability to set maximal and minimal sizes for
advanced units (size-min and size-max). This is useful if you want to avoid
runaway unit growth or units starving to death.

I have not dealt with the problems that were recently discussed on the list
yet, except for some preliminary moves, such as adding code for unit view
validation and clearance (still unused).

Hans