Add the handler code for incoming TFTP packets to udp6_input(),
and make sure that the TFTP code can send packets with both,
udp_output() and udp6_output() by introducing a wrapper function
called tftp_udp_output().

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