eth_mc_hash() was missing a return within a switch statement, and so
always falling through to the default case and returning failure.

EFI requires us to be able to specify the source address for
individual transmitted packets, and to be able to extract the
destination address on received packets.

Take advantage of this to rationalise the push() and pull() methods so
that push() takes a (dest,source,proto) tuple and pull() returns a
(dest,source,proto) tuple.

Multicast hashing is an ugly overlap between network and link layers.
EFI requires us to provide access to this functionality, so move it
out of ipv4.c and expose it as a method of the link layer.

Determine the network-layer packet type and fill it in for UNDI
clients.  This is required by some NBPs such as emBoot's winBoot/i.

This change requires refactoring the link-layer portions of the
gPXE netdevice API, so that it becomes possible to strip the
link-layer header without passing the packet up the network stack.

Achieved via Perl using:

perl -pi -e 's/pk_buff/io_buffer/g; s/Packet buffer/I\/O buffer/ig; ' \
	-e 's/pkbuff\.h/iobuf.h/g; s/pkb_/iob_/g; s/_pkb/_iob/g; ' \
	-e 's/pkb/iobuf/g; s/PKB/IOB/g;'

Added a large number of missing calls to free_pkb().  In the case of UDP,
no received packets were ever freed, which lead to memory exhaustion
remarkably quickly once pxelinux started up.

In general, any function with _rx() in its name which accepts a pk_buff
*must* either call free_pkb() or pass the pkb to another _rx() function
(e.g. the next layer up the stack).  Since the UDP (and TCP) layers don't
pass packet buffers up to the higher-layer protocols (the
"applications"), they must free the packet buffer after calling the
application's newdata() method.

Kill off the static single net device and move to proper dynamic
registration (which we need with the new device model).

Break the (flawed) assumption that all network-layer protocols can use
ARP; such network-layer protocols (i.e. IPv4) must now register as an ARP
protocol using ARP_NET_PROTOCOL() and provide a single method for checking
the existence of a local network-layer address.

implementation allows for only one, and does so without compromising on
the efficiency of static allocation).

Link-layer protocols are cleanly separated from the device drivers.

Network-layer protocols are cleanly separated from individual network
devices.

Link-layer and network-layer protocols are cleanly separated from each
other.

it with metadata in the pkb structure.  This is required since UNDI will
want to be able to parse the link-layer header without destroying it.