Make querystring variable name 'token' configurable.

'token' is a heavily impacted variable name, often used in security mechanisms. The default of token is probably okay, but it should be configurable.

My intent is to replace all references to $_REQUEST['token'] with a function call along these lines:

function tokenauth_url_token() {
  return $_REQUEST[variable_get('tokenauth_query_string', 'token')];
}

UI would be nice, but non-essential.