Function: mm-xemacs-find-mime-charset-1

Determine which MIME charset to use to send region as message.
This uses the XEmacs-specific latin-unity package to better handle the
case where identical characters from diverse ISO-8859-? character sets
can be encoded using a single one of the corresponding coding systems.

It treats `mm-coding-system-priorities' as the list of preferred
coding systems; a useful example setting for this list in Western
Europe would be '(iso-8859-1 iso-8859-15 utf-8), which would default
to the very standard Latin 1 coding system, and only move to coding
systems that are less supported as is necessary to encode the
characters that exist in the buffer.

Latin Unity doesn't know about those non-ASCII Roman characters that
are available in various East Asian character sets. As such, its
behavior if you have a JIS 0212 LATIN SMALL LETTER A WITH ACUTE in a
buffer and it can otherwise be encoded as Latin 1, won't be ideal.
But this is very much a corner case, so don't worry about it.