This was resulting in non-intuitive and surprising behaviour when queuing
commands. Removing this system results in Megaglest behaving more like
other RTS games.

For instance, the following are now possible and weren't previously:
- Queuing multiple attack ground commands to move along a specific path
attacking all enemies on the way.
- Queuing attack ground followed by hold position, which is a sensible
  thing to do if you want a unit to guard a specific area.
- Queuing a move command followed by a produce command, if you want to
  have your summoner summon a daemon at a specific location.

The behaviour of queuing the stop command is unchanged, and commands that
must be the last in the queue, such as morphing, are still properly replaced when something
else is queued.

 if(unitType != NULL && ct->getClass() == ccBuild)

for unit->computeCommandType

 to NetworkCommand class to prevent collision morph with build