bug#21771: 25.0.50; Can't customize whitespace-display-mappings

[Glenn Morris]

Ivan Andrus wrote:

>> This might be related to bug #2689,
>> http://debbugs.gnu.org/cgi/bugreport.cgi?bug=2689

> -  :valid-regexp "\\`.\\'"
> +  :valid-regexp "\\`\\(.\\|\n\\)\\'"

Note that this reintroduces part of the patch for #2689, which I had to
revert since it caused a worse problem (#3136) than the one it solved.
(I haven't checked whether the same issue applies now to your patch, but
you should.)