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/mhitu.c

@@ -2128,7 +2128,7 @@ doseduce(struct monst *mon)
                 /* have her call your gloves by their correct
                    name, possibly revealing them to you */
                 if (yourgloves)
-                    yourgloves->dknown = 1;
+                    observe_object(yourgloves);
                 verbalize("Well, then you owe me %s%s!",
                           yourgloves ? yname(yourgloves)
                                      : "twelve pairs of gloves",