Syatematic use of _main.cfg and _final.cfg: campaigns are now packages,

Warning: you will lose if (a) you are running under Windows, and (b) your 
compiler isn't GNU C or Borland C or something else with native dirent(3) 
support, and (c) our untested internal dirent(3) implementation is broken.

data/game.cfg

@@ -7,7 +7,7 @@
 {fonts.cfg}
 {english.cfg}
 
-{terrain-graphics.cfg}
+{terrain-graphics/}
 {units.cfg}
 {campaigns/}
 

data/terrain-graphics/_final.cfg

@@ -1,3 +1,5 @@
+# This file needs to be processt *after* all others in this directory
+#
 # The following flags are defined to have a meaning
 #
 # * base : the corresponding tile has already graphics for the terrain
@@ -12,8 +14,6 @@
 # the proper way to calculate the propabilities is described here
 # http://www.wesnoth.org/wiki/Terrain_Graphics_Probability
 
-{terrain-graphics}
-
 #-----------------------------------------------------------------
 # forest/pine<->Castle|Encampment special cases, also used by some other
 # terrain