IPv6: 6rd iproute2 support

IPv6: 6rd iproute2 support

This patch provides iproute2 facilities to configure 6rd tunnel. To
configure a 6rd tunnel, you need to configure a sit tunnel and set
6rd prefix as following :

  ip tunnel add sit1 mode sit local a.b.c.d ttl 64
  ip tunnel 6rd dev sit1 6rd-prefix xxxx:yyyy::/z

Optionally you can provide a relay prefix :

  ip tunnel 6rd dev sit1 6rd-relay_prefix e.f.g.h/i

Finally you can reset previous tunnel settings :

  ip tunnel 6rd dev sit1 6rd-reset

Signed-off-by: Alexandre Cassen <acassen@freebox.fr>