technically be necessary, because the "enable A20" command requires
only that the keyboard controller is ready to accept input (i.e. that
its input buffer is empty), and shouldn't also require that the
keyboard is ready to send output (i.e. that its output buffer is also
empty).  See http://www.smsc.com/main/tools/io-bios/42i.pdf section
3.1 ("Command Invocation") for a justification.

gateA20_set() is called on every real-mode transition (in case some
idiot piece of external code such as Intel's PXE stack decided it
would be fun to re-disable A20), so draining the keyboard buffer means
that we end up losing keypresses on some systems.  In particular, this
makes typing at the command line almost impossible, and causes
Etherboot to ignore Ctrl-Alt-Del.

We should really implement a gateA20_test() function to verify that
gate A20 has been correctly enabled, and think about adding other
commonly-used methods such as Fast Gate A20.