dir-locals.el: Adds White Space support

Trailing white spaces will be now always highlighted, not just in
prog-mode.

Also, the White Space package, which is available since GNU Emacs 22,
is loaded and activated locally in prog-mode.

Additionally, using White Space variables, we set highlighting through
faces on wrong indentation and the maximum length of a coding line.

Notice that:
 - The highlighting for the characters beyond the set length of a
   coding line is not activated by default, only for wrong
   indentations.
 - If the White Space package is not available, errors on loading or
   activation are ignored.
 - If the White Space mode is not activated the set variables would
   not have any effect.

v2: Removed too long lines trail highlighting, as suggested by Ilia
    Mirkin.

Signed-off-by: Andres Gomez <agomez@igalia.com>
Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>

.dir-locals.el

@@ -1,4 +1,5 @@
-((prog-mode
+((nil . ((show-trailing-whitespace . t)))
+ (prog-mode
   (indent-tabs-mode . nil)
   (tab-width . 8)
   (c-basic-offset . 3)
@@ -8,6 +9,10 @@
 	    (c-set-offset 'case-label '0)
 	    (c-set-offset 'innamespace '0)
 	    (c-set-offset 'inline-open '0)))
-  )
+  (whitespace-style face indentation)
+  (whitespace-line-column . 79)
+  (eval ignore-errors
+        (require 'whitespace)
+        (whitespace-mode 1)))
  (makefile-mode (indent-tabs-mode . t))
  )