Accurately track which items have been discovered, even if not #named

This fixes a couple of bugs: a long-standing bug in which writing a
scroll by label could fail even if you've already seen a scroll with
that label (due to the game not tracking whether or not you've seen a
scroll if it doesn't have a name); and a somewhat newer bug in which
spellbooks auto-identified by Wizard knowledge were marked as having
been encountered (rather than as known but not encountered).

Breaks save file compatibility, but not bones files.

src/spell.c

@@ -234,7 +234,7 @@ deadbook(struct obj *book2)
 
     You("turn the pages of the Book of the Dead...");
     makeknown(SPE_BOOK_OF_THE_DEAD);
-    book2->dknown = 1; /* in case blind now and hasn't been seen yet */
+    observe_object(book2); /* in case blind now and hasn't been seen yet */
     /* KMH -- Need ->known to avoid "_a_ Book of the Dead" */
     book2->known = 1;
     if (invocation_pos(u.ux, u.uy) && !On_stairs(u.ux, u.uy)) {
@@ -896,8 +896,9 @@ skill_based_spellbook_id(void)
         }
 
         if (objects[booktype].oc_level <= known_up_to_level)
-            /* makeknown(booktype) but don't exercise Wisdom */
-            discover_object(booktype, TRUE, FALSE);
+            /* makeknown(booktype) but don't exercise Wisdom or mark as
+               encountered */
+            discover_object(booktype, TRUE, FALSE, FALSE);
     }
 }