usbdump and hw.usb.no_pf

[Warren Block]

A 9-STABLE desktop running amd64 steadfastly refuses to run usbdump(8):

# usbdump -i usbus0 -v -w /tmp/usb.log
read: Device not configured

0 packets captured
0 packets received by filter
0 packets dropped by kernel

It turns out that having hw.usb.no_pf="1" set in /boot/loader.conf to 
speed up booting disables usbdump.  Interestingly, this sysctl can be 
changed at runtime, but usbdump will still not work.  The value has to 
be set in /boot/loader.conf at boot for usbdump to work again.

If usbdump could produce a warning when this sysctl is was set to 1 at 
boot, it would make the mysterious error less mysterious.