Patches to improve Hurd parts: dup3

[Roland McGrath]

> Hello!
> 
> On Wed, Dec 17, 2008 at 07:54:04PM -0800, Roland McGrath wrote:
> > __dup2 has to call __dup3, not dup3.
> 
> There is no such symbol, cf.:

Well, you need one in the hurd definition.

> sysdeps/unix/syscalls.list:
>     # File name     Caller  Syscall name    Args    Strong name     Weak names
>     [...]
>     dup3            -       dup3            i:iii   dup3
> 
> That's why I also named it dup3 in the new sysdeps/mach/hurd/dup3.c.

There is no other caller of dup3 in the non-hurd libc code,
so it doesn't need one.  Hurd does.