[netdevice] Allow per-device receive queue processing to be frozen

[netdevice] Allow per-device receive queue processing to be frozen

Several use cases (e.g. the UNDI API and the EFI SNP API) require
access to the raw network device receive queue, and so currently use
manual calls to netdev_poll() on a specific network device in order to
prevent received packets from being processed by the network stack.

As an alternative, provide a flag that allows receive queue processing
to be frozen on a per-device basis.  When receive queue processing is
frozen, packets will be enqueued as normal, but will not be
automatically dequeued and passed up the network stack.

Signed-off-by: Michael Brown <mcb30@ipxe.org>