bug#15925: 24.3.50; error when customizing whitespace-display-mappings

[Drew Adams]

> I very much hope we leave the ^M and ^L display alone.  This is what
> we did since day one (and yes, \n is a special case), and I'd rather
> we didn't change that just in the name of consistency.

+1.  But why not for all control chars?

I haven't been following this thread - no
doubt I'm not seeing special cases etc.
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Currently, `ctl-arrow' controls the display.
That works for me.

And any code that changes the display-table
entry of a char has its intended effect.
Also good.
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(Maybe `ctl-arrow' could be extended, to allow
different non-nil values to specify different
representations, including \n and ?\n.  For
compatibility, nil would still specify octal.)