[Patch] Use variadic templates for tr1::is_function

[Paolo Carlini]

... but then why we have:

  /**
   * @if maint
   * Retrieve the result type for a function type.
   * @endif
   */
  template<typename _Res, typename... _ArgTypes>
    struct _Weak_result_type_impl<_Res(_ArgTypes...)>
    {
      typedef _Res result_type;
    };

in the first place? Does it ever trigger? (Still, the other 
specializations, for function pointers and references, should be added 
of a version for variadic functions, I think)

Paolo.