netdev: Clean up and refactor packet receive interface.

netdev: Clean up and refactor packet receive interface.

The Open vSwitch tree only has one user of the ability for a netdev to
receive packets from a network device.  Thus, this commit simplifies the
common-case use of the netdev interface by replacing the "ethertype" option
from "struct netdev_options" by a new netdev_listen() call.

The only user of netdev_listen() wants to receive all packets from a
network device, so this commit also removes the ability to restrict the
received packets to a particular protocol.  (This ability was once used by
the Open vSwitch integrated DHCP client, but that code has been removed.)

This commit also simplifies and improves the implementation of the code
in netdev-linux that started listening to a network device.  Before, I had
not figured out how to avoid receiving all packets on all devices before
binding to a particular device, but I took a closer look at the kernel code
and figured it out.

I've tested that the userspace datapath (dpif-netdev), the only user of
netdev_recv(), still works after this change.