fix github issue #1278 - spellbook discovery

Issue reported by ars3niy:  having a pauper wizard #enhance bare-handed
fighting to basic caused the hero to learn all level 1 spellbooks.

Spellbook discovery for wizards when advancing skills is intentional,
to replace the old Luck-based chance of writing unknown books with
magic markers.  (The Luck bias for wizards still applies for scrolls.)

Discovering spellbooks when advancing non-spell skills does feel wrong.
Change it to only happen when advancing spell skills.

This isn't pauper-specific, it's just a lot more noticeable in that
state.

There may be better ways to cope with this, but for the time being I'm
marking the issue 'fixed'.

Fixes #1278

src/weapon.c

@@ -1148,7 +1148,11 @@ skill_advance(int skill)
         P_SKILL(skill) >= P_MAX_SKILL(skill) ? "most" : "more",
         P_NAME(skill));
 
-    skill_based_spellbook_id();
+    /* wizards discover spellbook IDs depending on spell 'school' skill limits;
+       this allows them to successfully write books for unknown spells without
+       the Luck bias they used to have over other roles */
+    if (skill >= P_FIRST_SPELL && skill <= P_LAST_SPELL)
+        skill_based_spellbook_id();
 }
 
 static const struct skill_range {