[latin-unity] Fix suboptimal interaction of latin-unity and VM.

[Stephen J. Turnbull]

APPROVE COMMIT latin-unity

I'll be keeping an eye on this, but according to Fabrice and Joachim
life is much better.  See <3b564at2.fsf at esemetz.metz.supelec.fr> and
<17877.35058.979710.474896 at puma.r.npc.de>.

Thanks to Fabrice and Joachim for instant response to my request for
testing, and to Aidan for his advice on the basic approach.

 > +	* latin-unity.el (latin-unity-representations-present-region):
 > +	Assume C1 characters map to themselves, as is true for all
 > +	ISO-8859-X coding systems.  Should fix the bug reported by
 > +	Joachim Schrod and Fabrice Popineau.  See
 > +	<ire6dxp3.fsf at esemetz.metz.supelec.fr>.