Improve network device scanning in the netdev module. First, make it use the
`device.subr' framework (improving performane and reducing sub-shells). Next
improve the `device.subr' framework itself. Make use of the `flags' device
struct member for network interfaces to indicate if an interface is Active,
Wired Ethernet, or 802.11 Wireless. Functions have been added to make checks
against the `flags' bit-field quick and efficient. Last, add function for
rescanning the network to update the device registers. Remove an unnecessary
local (ifn) while we're here (use already provided local `if').