/*      change_user(user=nil, passwd=nil, db=nil)    */
static VALUE change_user(int argc, VALUE* argv, VALUE obj)
{
    VALUE user, passwd, db;
    char *u, *p, *d;
    MYSQL* m = GetHandler(obj);
    rb_scan_args(argc, argv, "03", &user, &passwd, &db);
    u = NILorSTRING(user);
    p = NILorSTRING(passwd);
    d = NILorSTRING(db);
    if (mysql_change_user(m, u, p, d) != 0)
        mysql_raise(m);
    return obj;
}