default_value defaults to nullptr.

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.

Don't abort the process, let the caller decide instead.

Implement a fast path for UTF-8 which leaves fs_charset empty, and
don't assign a value to fs_charset if there's no configuration.