/* call-seq:
 *   krb5.change_password(old, new)
 *
 * Changes the password for the principal from +old+ to +new+. The principal
 * is defined as whoever the last principal was authenticated via the
 * Krb5#get_init_creds_password method.
 *
 * Attempting to change a password before a principal has been established
 * will raise an error.
 *
 * Example:
 *
 * krb5.get_init_creds_password('foo', 'XXXXXX') # Authenticate 'foo' user
 * krb5.change_password('XXXXXX', 'YYYYYY')      # Change password for 'foo'
 */
static VALUE rkrb5_change_password(VALUE self, VALUE v_old, VALUE v_new){