Find out current locale charset. The function respect the CHARSET
environment variable, but typically uses nl_langinfo(CODESET) when
it is supported. It fall back on "ASCII" if CHARSET isnt set and
nl_langinfo isnt supported or return anything.
Note that this function return the applications locales preferred
charset (or threads locales preffered charset, if your system
support thread-specific locales). It does not return what the
system may be using. Thus, if you receive data from external
sources you cannot in general use this function to guess what
charset it is encoded in. Use stringprep_convert from the external
representation into the charset returned by this function, to have
data in the locale encoding.
Return value: Return the character set used by the current locale.
It will never return NULL, but use "ASCII" as a fallback.