hotplug notification

[Alexander Polakov <polachok@xxxxxxxxx>]

I think it could be nice to receive notification for creation and
removal of device nodes in userland. So I ported hotplug(4) and
hotplugd(8) from OpenBSD. I like the design, it's clean and simple:
an event contains type (attach or detach), device class (disk, tty or
generic) and name (as in /dev). Events are queued and can be read
from /dev/hotplug one-by-one (hotplugd(8) does that and can be
configured to execute commands on event arrival).

You can get code to review here:
    http://gitweb.dragonflybsd.org/~polachok/dragonfly.git/hotplug2