We don't need this anymore: Win32 doesn't use this library at all, and
everything else has either iconv() or libicu.

default_value defaults to nullptr.

I suppose there is no non-WIN32 operating system that doesn't have
fnmatch().  So let's remove the GLib implementation.

The default is always UTF-8.

Don't define HAVE_FS_CHARSET, and make GetFSCharset() return "ACP"
instead of "UTF-8".  Ignore the configuration setting, which had no
effect anyway.

Use wchar_t for everything on Windows.  Solves a lot of filesystem
charset problems.

Don't use FILE_ATTRIBUTE_NORMAL, it's a "magic" value for something
else.  To check if a file is a regular file, we need to check if it's
NOT a directory (or a device).