Parses all scanned wireless network cells.

@private
@static
@category iwlist
@param {function} callback The callback function.
function parse_scan(show_hidden, callback) {
  return function(error, stdout, stderr) {
    if (error) callback(error);
    else
      if (show_hidden) {
        callback(error, stdout
        .split(/Cell [0-9]+ -/)
        .map(parse_cell)
        .filter(has_keys)
        .sort(by_signal));
    } else {
        callback(error, stdout
        .split(/Cell [0-9]+ -/)
        .map(parse_cell)
        .filter(has_ssid)
        .sort(by_signal));
    }
  };
}