fix #H7065 - clairvoyance shows trap over monsters

and over the hero.  3.6.0's clairvoyance tried to show things in
a non-standard sequence, which was intentional but had unintended
side-effects like the disappearing monsters complained about in the
report.  To make it work as intended would have required --More--
whenever it kicked in, which is much too intrusive when it happens
every N turns rather than when explicitly casting the spell.

Redo it substantially, and give preference to monsters over objects,
objects over traps, and traps over underlying terrain like normal
vision-based display does.  It now detects all monsters within its
bounding box but shows ones which aren't directly in view as
"unseen monster" unless via spell cast at skilled or expert, or at
basic when also having intrinsic clairvoyance.

src/pager.c

@@ -895,12 +895,17 @@ struct permonst **for_supplement;
     }
 
     if (sym == DEF_INVISIBLE) {
+        extern const char altinvisexplain[]; /* drawing.c */
+        /* for active clairvoyance, use alternate "unseen creature" */
+        boolean usealt = (EDetect_monsters & I_SPECIAL) != 0L;
+        const char *unseen_explain = !usealt ? invisexplain : altinvisexplain;
+
         if (!found) {
-            Sprintf(out_str, "%s%s", prefix, an(invisexplain));
-            *firstmatch = invisexplain;
+            Sprintf(out_str, "%s%s", prefix, an(unseen_explain));
+            *firstmatch = unseen_explain;
             found++;
         } else {
-            found += append_str(out_str, an(invisexplain));
+            found += append_str(out_str, an(unseen_explain));
         }
     }