Make show-paren-match face visible on mono-color displays

* lisp/faces.el (show-paren-match): Use the underline face for
mono-color displays.  (Bug#21481)

lisp/faces.el

@@ -2691,10 +2691,12 @@ It is used for characters of no fonts too."
      :background "turquoise")		; looks OK on tty (becomes cyan)
     (((class color) (background dark))
      :background "steelblue3")		; looks OK on tty (becomes blue)
-    (((background dark))
+    (((background dark) (min-colors 4))
      :background "grey50")
+    (((background light) (min-colors 4))
+     :background "gray")
     (t
-     :background "gray"))
+     :inherit underline))
   "Face used for a matching paren."
   :group 'paren-showing-faces)