"rep movsb".

in a long loop before giving up on them.  Record method which worked
and default to that method on next attempt.

etherboot.h is only now used by drivers that have not been updated to
the new API.

Kill off use of etherboot.h outside drivers/net.

optimisation of only trying to do so when necessary.

number of (potentially very slow) gateA20_set operations.

Die with a fatal error if we are unable to set gate A20; if this fails
then we are bound to experience memory corruption at a later stage,
and I'd prefer to pick it up early.

in tcp_rx_data().

Clarify comments on discarding duplicate or out-of-order data.

the UNDI stack.

Ignore obviously invalid length combinations (as returned by
e.g. VMWare's PXE stack).

Limit to one packet per poll to avoid memory exhaustion.

Always send EOI; do not chain to BIOS's default interrupt handler.
They are just too unpredictable; at least VMware's seems to kill the
machine if you go anywhere near it.

Disable interrupts after return from PXENV_UNDI_ISR, just in case some
dumb PXE stack enables them.