Don't show corridors as lit in #terrain

Because back_to_glyph assumes the hero can directly see the spot, when
used for #terrain (e.g. when a spot was covered by a remembered object)
it would display corridors as lit if lit_corridor was enabled.  Instead
of trying to suppress back_to_glyph's S_litcorr result only under
circumstances where it would be unusual, just show all corridor spots
with S_corr, so that none of them appear as "lit corridor".  This
is consistent with what is already done for room spots for #terrain
(S_darkroom is forced to the basic S_room across the board).

src/detect.c

@@ -1981,8 +1981,11 @@ reveal_terrain_getglyph(coordxy x, coordxy y, int full, unsigned swallowed,
             }
         }
     }
+    /* FIXME: dirty hack */
     if (glyph == cmap_to_glyph(S_darkroom))
-        glyph = cmap_to_glyph(S_room); /* FIXME: dirty hack */
+        glyph = cmap_to_glyph(S_room);
+    else if (glyph == cmap_to_glyph(S_litcorr))
+        glyph = cmap_to_glyph(S_corr);
     return glyph;
 }