quotearg: do not depend on localcharset

quotearg: do not depend on localcharset

* lib/quotearg.c: Do not include localcharset.h.
(gettext_quote): Do not use locale_charset; instead, use
mbrtoc32 to determine whether the character set uses UTF-8.
Treat GB-18030 like similar encodings, as it is not worth the
trouble of special-casing: its special-case code has been broken
since 2011, nobody cared enough to report the bug, and the idea
that someone would use GB-18030 without translations is weird anyway.
* modules/quotearg (Depends-on): Remove localcharset.