tcpdump to filter wlan frame type/subtype

[Sepherosa Ziehau]

Hi all,

Following patch makes tcpdump filter 802.11 frames based on
type/subtype.  You can now:
tcpdump -ni wlan_iface wlan subtype assoc-req
to grab association requests, instead of searching them in piles of
beacons.  The patched tcpdump man page will show you more details
about the type/subtype syntax.
http://leaf.dragonflybsd.org/~sephe/tcpdump_wlan.diff
Please review it.  If no objection comes, I will commit it one week later.
Best Regards,
sephe
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