Autodiscovery hook for node.module

Allows other modules to insert autodiscovery code (e.g. FOAF, ATOM, RSD) on main page

modules which wants to insert autodiscovery code will have to provide module_default_node() that does drupal_set_html_head.

e.g. blogapi.module can have the the following function to provide autodiscovery of its xmlrpc engine vis RSD

function blogapi_default_node() {
  drupal_set_html_head('<link rel="EditURI" type="application/rsd+xml" title="RSD" href="' . url('blogapi/rsd',NULL,NULL,TRUE) . '" />');
}