netroot: add common handler for network root devices

netroot: add common handler for network root devices

/sbin/netroot is a jumping off point to allow various network
root devices to share infrastructure. It will loop over scriptlets
in the netroot handler, looking for a handler to run for this type
of netroot. Handlers can do choose to act based on command line
options to the kernel, or via DHCP options received on this interface.
They should massage root= into a form suitable for their handler.