Give two more uses to dipping in potion of restore ability

It was already possible to dip an eroded object in restore ability to
remove the erosion. This commit adds two more things it can do:
- Increasing an item's negative enchantment or charge, to a maximum of
  0, provided it wasn't eroded and got that fixed instead.
- Restoring the ink in a spellbook to maximum freshness. (I don't see
  any additional potential for abuse here; in the case where you refresh
  spell knowledge 4 times and want even more from the same book, you
  are probably far into the game and are likely to have either found a
  duplicate spellbook, have a magic marker, or are doing large-scale
  farming.)

This commit also restructures the code around restore ability dipping;
in particular, enshrining a principle that cursed restore ability dips
will just do nothing (not even use up the potion) and fall into the
"Interesting..." case, similar to how cursed restore ability has no
effect when quaffed.

Author: copperwater

doc/xnh-changelog-10.0.md

@@ -24,6 +24,12 @@ changes:
 
 - The Ides of March (March 15th) is recognized as one of the in-game holidays,
   and slime molds will generate as "Caesar salads" on that day.
+- Items with negative enchantment or charges can be dipped in a noncursed potion
+  of restore ability to raise their enchantment, to 0 if the potion is blessed
+  or by 1 point if not. If the item is eroded, that will be fixed first and a
+  second potion will be required to address any negative enchantment.
+- Spellbooks can be dipped in a noncursed potion of restore ability to fully
+  refresh the ink on their pages.
 
 ### Interface changes
 

src/potion.c

@@ -3020,14 +3020,52 @@ potion_dip(struct obj *obj, struct obj *potion)
         return ECMD_TIME;
     }
 
-    /* removing erosion from items */
-    if (potion->otyp == POT_RESTORE_ABILITY && !potion->cursed
-        && erosion_matters(obj) && (obj->oeroded || obj->oeroded2)) {
-        obj->oeroded = obj->oeroded2 = 0;
-        pline("%s as good as new!", Yobjnam2(obj, Blind ? "feel" : "look"));
-        if (potion->dknown)
+    /* Restore ability has many uses.
+     * Like with quaffing it, it generally doesn't have any negative effects
+     * if cursed; it just won't do anything.
+     * If it could do multiple things to an item (rusty -1 weapon for instance),
+     * it will only fix the first case. */
+    if (potion->otyp == POT_RESTORE_ABILITY && !potion->cursed) {
+        boolean did_something = FALSE;
+        boolean learn_it = FALSE;
+
+        /* removing erosion from items */
+        if (erosion_matters(obj) && (obj->oeroded || obj->oeroded2)) {
+            obj->oeroded = obj->oeroded2 = 0;
+            pline("%s as good as new!", Yobjnam2(obj, Blind ? "feel" : "look"));
+            learn_it = TRUE;
+            did_something = TRUE;
+        }
+        /* undoing a negative enchantment */
+        else if (spe_means_plus(obj) && obj->spe < 0) {
+            obj->spe = (potion->blessed ? 0 : obj->spe + 1);
+            pline("%s %smore effective.",
+                  Yobjnam2(obj, Blind ? "feel" : "look"),
+                  obj->spe < 0 ? "a little " : "");
+            if (obj->known)
+                learn_it = TRUE;
+            else
+                trycall(potion);
+            did_something = TRUE;
+        }
+        /* refreshing a faded spellbook */
+        else if (obj->oclass == SPBOOK_CLASS
+                 && obj->otyp != SPE_BOOK_OF_THE_DEAD
+                 && obj->otyp != SPE_BLANK_PAPER && obj->otyp != SPE_NOVEL
+                 && obj->spestudied > 0) {
+            obj->spestudied = 0;
+            if (Blind)
+                pline("The pages of %s feel crisper.", yname(obj));
+            else
+                pline("The ink in %s becomes sharp and fresh again!",
+                      yname(obj));
+            learn_it = TRUE;
+            did_something = TRUE;
+        }
+        if (learn_it && potion->dknown)
             makeknown(POT_RESTORE_ABILITY);
-        useup(potion);
+        if (did_something)
+            useup(potion);
         return ECMD_TIME;
     }