Let the difficulty setting of Jebel Barkal proportionally influence the number of attackers spawned, refs #4963.
Remove iberian starting walls but keep the towers as requested by lobby players.
Update mappreview to include the walls from rP21556.
Use Basic rank, since Gaia doesn't have a TechnologyManager and since the rank bonuses are autoresearched-techs
(noted by temple recently and FeldFeld in a match before the first version of D204).
git-svn-id: https://svn.wildfiregames.com/public/ps/trunk@21631 3db68df2-c116-0410-a063-a993310a9797