IPFW MT_TAG --> m_tag conversion

[User Joerg]

Hi all,
if you use IPFW, please test
http://leaf.dragonflybsd.org/~joerg/ipfw_no_mt_tag.diff
for any regressions beside speed. This is a pre-requirement
to get it IPFW module-loadable and cleaner integrated into
the network stack. Again, ATM I don't worry about a slightly
reduced performance here.

Joerg