slirp: Add support for stateless DHCPv6

slirp: Add support for stateless DHCPv6

Provide basic support for stateless DHCPv6 (see RFC 3736) so
that guests can also automatically boot via IPv6 with SLIRP
(for IPv6 network booting, see RFC 5970 for details).

Tested with:

    qemu-system-ppc64 -nographic -vga none -boot n -net nic \
        -net user,ipv6=yes,ipv4=no,tftp=/path/to/tftp,bootfile=ppc64.img

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>