[DCR] Remove devstat

[Sue Howard]

Hi,

I found devstat is not widely used in the device drivers. Based the
grep result of 'devstat_new_entry', it shows only cam, ata, fd,
geom_disk are using it. In my workstation, the output of 'sysctl
kern.devstat' shows:

kern.devstat.version: 6
kern.devstat.generation: 229
kern.devstat.numdevs: 2

I am wondering if devstat can be replaced by the sysctl nodes %desc,
%pnpinfo etc that jhb introduced. Almost all the information devstat
can provide can be also exposed by such sysctl node.

I have plan to introduce a new node named status so that userland can
disable on individual device. Before that, I want to understand some
related stuffs.

Any comments about this?

Howard Sue