Use fixed-pitch font for display-line-numbers

* lisp/faces.el (line-number): Use a fixed-pitch font by default,
even if the default face uses a variable-pitch font.  Reported by
James Cloos <cloos@jhcloos.com>.

lisp/faces.el

@@ -2465,9 +2465,12 @@ If you set `term-file-prefix' to nil, this function does nothing."
  :version "21.1"
  :group 'basic-faces)
 
-;; Definition stolen from linum.el.
+;; Definition originally stolen from linum.el.
+;; The monospace part is so we don't accidentally display numbers
+;; using a variable-pitch font just because the default face uses
+;; such a font.
 (defface line-number
- '((t :inherit (shadow default)))
+ '((t :inherit (shadow default) :family "Monospace Serif"))
  "Face for displaying line numbers.
 This face is used when `display-line-numbers' is non-nil.