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.

include/sp_lev.h

@@ -145,6 +145,7 @@ typedef struct {
     schar peaceful, asleep;
     short female, invis, cancelled, revived, avenge, fleeing, blinded,
         paralyzed, stunned, confused, waiting;
+    short m_lev_adj;
     long seentraps;
     short has_invent;
     mmflags_nht mm_flags; /* makemon flags */