netdev: New function netdev_nodev_get_flags().

netdev: New function netdev_nodev_get_flags().

An upcoming change will move detection of netdev flags changes into
secchan.  That change will require polling the flags for many network
devices.  Opening and closing that many network devices, or keeping
around fds for them, is a waste of time (or fds).

But it's easy to get flags for any number of netdevs from a single file
descriptor, so add a function to do this.