switch dymmynet from mbuf based tagging to m_tag

[Sepherosa Ziehau]

Hi all,

Followin patcch switches dymmynet from mbuf based tagging to m_tag:
http://leaf.dragonflybsd.org/~sephe/dummynet_mtag.diff

Please review it.  If no objection comes, it will committed three days later.

Best Regards,
sephe

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