Jquery Ctrl + Click sem a tecla control

Permitir múltipla selecção num select multiple sem uso da tecla control.

by Salustiano Silva

HTML

<select size="4" multiple>
    <option>1</option>
    <option>2</option>
    <option>3</option>
</select>
<p></p>

JavaScript

var multiSelect = {};
function init() {      
  var s = document.getElementsByTagName('select');
  for (var i = 0; i < s.length; i++) {
    if (s[i].multiple) {
      var n = s[i].name;
      multiSelect[n] = [];
      for (var j = 0; j < s[i].options.length; j++) {
        multiSelect[n][j] = s[i].options[j].selected;
      }
      s[i].onclick = changeMultiSelect;
    }
  }
}
function changeMultiSelect() {
  var n = this.name;
  for (var i=0; i < this.options.length; i++) {
    if (this.options[i].selected) {
      multiSelect[n][i] = !multiSelect[n][i];
    }
    this.options[i].selected = multiSelect[n][i];
  }
}
window.onload = init;