Implement dkstats for worm devices so they show up in iostat et al.

Implement dkstats for worm devices so they show up in iostat et al.
I got tired of not seeing my worm stats show up during a burn. :)
[Joerg, I just stapled in 1MB/sec for a bogus xfer rate and left seek = 1,
 as suggested - I'm not going to dynamically calculate the xfer rate from
 a known device spectable, OK? :-)]
Reviewed by:	joerg