I preparation for TSO in software, cleanup the fragment code. Use

I preparation for TSO in software, cleanup the fragment code.  Use
if_output_ml() to send mbuf lists to interfaces.  This can be used
for TSO, fragments, ARP and ND6.  Rename variable fml to ml.  In
pf_route6() split the if else block.  Put the safety check (hlen +
firstlen < tlen) into ip_fragment().  It makes the code correct in
case the packet is too short to be fragmented.  This should not
happen, but other functions also have this logic.
No functional change.  OK sashan@