replace callout with systimer for dummynet

[Sepherosa Ziehau]

Hi all,

Following patch replace callout with systimer for dummynet:
http://leaf.dragonflybsd.org/~sephe/dummynet0.diff

This eliminates yet another dependency on system HZ, so when switching
it to use mtag, I won't need to increase my test boxes HZ :)

Please review it.  If no objection comes, I will commit it two days later.

Best Regards,
sephe

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