Validate above-Latin1 characters in \N{} aliases

Validate above-Latin1 characters in \N{} aliases

This completes the process of allowing users to define their own aliases
for \N{} in any language they choose.  Names have some validation
applied so that they can't, for example, begin with something that is a
digit in some Unicode script.  Tests and documentation are included in
this patch.  The loop in toke.c that does the validation for
user-supplied translators is revamped, and the messages that are output
when there is an error are fixed to work with UTF-8.