fix engraving with miscellaneous items

Reported directly to devteam:  when selecting an item to engrave
with, "getobj refactor"'s stylus_ok() classified none-of-the-above
objects as not viable instead of allowing arbitrary things to
engrave in the dust or give specific can't-use-that messages like
for food.  Make the suggested fix:  change default result when
checking an inventory item for applicability from GETOBJ_EXCLUDE
to GETOBJ_DOWNPLAY.

Engrave feedback rather than getobj selection:  splash of venom
isn't necessarily wizard mode-only so don't reject attempting to
engrave with it as using an illegal object.  And remove the extra
question mark from its "poison pen" message.

src/engrave.c

@@ -442,7 +442,7 @@ stylus_ok(struct obj *obj)
         && (obj->otyp == TOWEL || obj->otyp == MAGIC_MARKER))
         return GETOBJ_SUGGEST;
 
-    return GETOBJ_EXCLUDE;
+    return GETOBJ_DOWNPLAY;
 }
 
 /* can hero engrave at all (at their location)? */
@@ -879,11 +879,12 @@ doengrave(void)
         break;
 
     case VENOM_CLASS:
-        if (wizard) {
-            pline("Writing a poison pen letter??");
-            break;
-        }
-        /*FALLTHRU*/
+        /* this used to be ``if (wizard)'' and fall through to ILLOBJ_CLASS
+           for normal play, but splash of venom isn't "illegal" because it
+           could occur in normal play via wizard mode bones */
+        pline("Writing a poison pen letter?");
+        break;
+
     case ILLOBJ_CLASS:
         impossible("You're engraving with an illegal object!");
         break;