/**
 *
 * Scanner::add_charset
 *
 * Register a new charset encoding to be used.
 *
 * Throws a RuntimeError if LMS is unable to add the charset.
 *
 * * p1 (charset) charset name as understood by iconv_open(3).
 *
 * * return charset
 *
 */
static VALUE scanner_add_charset(VALUE self, VALUE charset) {