arbitrary commands when devices come and go in the device tree (which is
different than the /dev directory).
This is an initial version. Much of the planned power isn't here.
Instead of doing the full matching, we always run /etc/devd-generic.
/etc/devd.generic will go away at some point, I think.
I'm committing it in this early state so I can start getting feedback
from early adapters.
Approved by: re
