Allow monster level adjustments in special levels

Add a new parameter to des.monster, m_lev_adj, which is a level
adjustment for the monster.  This only applies to the monster's
level, so basically only affects the spellcasting, it does not
change the monster's hit die or inventory.

Change one of the shamans in Orctown to be 3 levels higher.

dat/minetn-1.lua

@@ -130,8 +130,10 @@ for i=1,5 + math.random(1 - 1,1*10) do
    end
 end
 -- shamans can be hanging out in/near the temple
-for i=1,math.random(2 - 1,2*3) do
-   des.monster({ id = "orc shaman", coord = near_temple:rndcoord(0), peaceful=0 });
+-- one of the shamans is higher level
+for i=1,math.random(1,6) do
+   des.monster({ id = "orc shaman", coord = near_temple:rndcoord(0), peaceful=0,
+                 m_lev_adj = (i == 1) and 3 or 0 });
 end
 -- these are not such a big deal
 -- to run into outside the bars