System.TM timezone confusion

[ceder (-) Per Cederqvist @ Pike (-) developers forum]

Untested fix:

> diff --git a/src/builtin.cmod b/src/builtin.cmod
> index 284a7c453f..158952f0bc 100644
> --- a/src/builtin.cmod
> +++ b/src/builtin.cmod
> @@ -565,6 +565,7 @@ PIKECLASS TM
>          THIS->t.tm_isdst = -1;
>          THIS->t.tm_mday = 1;
>          THIS->unix_time = 0;
> +        SET_ZONE(THIS, "UTC");
>          THIS->modified = 1;
>      }
>