Get the list of children pids of the given pid.
@public
@param  {Number|String} pid A PID. If -1 will return all the pids.
@param  {Object} [options] Optional options object.
@param  {Boolean} [options.root=false] Include the provided PID in the list.
@param  {Boolean} [options.advanced=false] Returns a list of objects in the
format {pid: X, ppid: Y}.
@param  {Function} [callback=undefined] Called when the list is ready. If not
provided a promise is returned instead.
@returns  {Promise.<Object[]>} Only when the callback is not provided.
function list(pid, options, callback) {
  if (typeof options === 'function') {
    callback = options;
    options = undefined;
  }

  if (typeof callback === 'function') {
    pidtree(pid, options, callback);
    return;
  }

  return pify(pidtree, pid, options);
}