add an ipv6 "don't fragment" flag to mbufs for ip6_output to use.

add an ipv6 "don't fragment" flag to mbufs for ip6_output to use.

if you need to send an ipv6 packet with ip6_send(), there's no DF
bit in an ipv6 packet and no way to pass the ip6 options to ip6_output
to tell it to not allow fragmentation. this adds an M_IPV6_DF_OUT
"checksum" flag so something creating ipv6 packets a long way from
ip6_output can easily tell it to not allow fragmentation.

grumbling and ok claudio@