quotearg: fall back to Unicode single quotes in UTF-8 and GB-18030 locales

quotearg: fall back to Unicode single quotes in UTF-8 and GB-18030 locales

Most programs do not have translation catalogs for English and much less
separate catalogs for British and American English.  Drop the suggestion
to translators about these two, and provide it automatically for Unicode
locales.  Like most programs, even those using American English, we use
single quotation marks.  This conflicts with the American typographic
convention, but works better when you cite the entire error message
within double quotes.  It also tries not to clash with established
practice and with what non-gnulib programs will usually do.

* lib/quotearg.c (gettext_quote): Hard-code U+2018 and U+2019 when using
an UTF-8 or GB-18030 locale.  The list of other locales with quotes was
provided by Bruno Haible.
(quotearg_buffer_restyled): Adjust instructions to translators.
* lib/quotearg.h (locale_quoting_style): Do not put an example in the
text, since this would be wrong when using Unicode.
* modules/quotearg: Depend on c-strcaseeq.