Some spell code reorg

Keep the internal spell array index inside spell.c,
and refer to spells outside of it with the otyp id.

include/spell.h

@@ -6,6 +6,7 @@
 #define SPELL_H
 
 #define NO_SPELL 0
+#define UNKNOWN_SPELL (-1)
 
 /* spellbook re-use control; used when reading and when polymorphing */
 #define MAX_SPELL_STUDY 3