Wizards learn about spellbooks as they enhance their spell skills

Previously, Wizards got a boost to the chance of writing unknown
spellbooks based purely on being a Wizard (with the chance still
luck-based), leading to a very large power spike when the Wizard
gained access to a luckstone and the ability to max out luck.
This had two main issues: this power spike came *after* the major
early-game difficulty spike, often leaving Wizards forced to deal
with it without having appropriate spells; and it promotes
grinding (for Luck and for Magicbane) at an early point in the
game, meaning that the Wizard early game effectively followed a
sequence of extreme difficulty -> grinding -> minimal difficulty,
which isn't very good balance-wise.

With this commit, Wizards lose their advantage to writing unknown
spellbooks by guessing, and instead learn spellbook IDs based on
their spell skills (advancing a skill gives knowledge of higher-
level spellbooks). This means that writing unknown spellbooks
becomes guaranteed with sufficient skill, but has no advantage
over non-Wizards in schools where the Wixard does not have
sufficient skill.

Due to Wizards' skill caps, there are two spells which they can't
ever write guaranteed: create familiar and charm monster. Create
familiar is a fairly niche spell (that doesn't match the Wizard
playstyle that well) and being unable to write it is not a major
problem. The inability to easily write charm monster is
intentional.

src/weapon.c

@@ -1136,6 +1136,8 @@ skill_advance(int skill)
     You("are now %s skilled in %s.",
         P_SKILL(skill) >= P_MAX_SKILL(skill) ? "most" : "more",
         P_NAME(skill));
+
+    skill_based_spellbook_id();
 }
 
 static const struct skill_range {
@@ -1689,6 +1691,8 @@ skill_init(const struct def_skill *class_skill)
     /* each role has a special spell; allow at least basic for its type
        (despite the function name, this works for spell skills too) */
     unrestrict_weapon_skill(spell_skilltype(gu.urole.spelspec));
+
+    skill_based_spellbook_id();
 }
 
 void