A FUZIX target for z80pack.

Test environment for a very squashed build to see what we can achieve in
64K

We build a kernel from 0x8000-0xFFFF and everything needs to either fit in
that space, or we will need some kind of overlay for obscure syscalls. We
can't however swap the overlays over userspace because there may be pointers
to userspace addresses in the syscalls. One option might be to mark binaries
by required level and have level 1 binaries given less room but loaded with
the overlay. That would work well since it's only a few obscure calls we need
to boot out.

The obvious calls to bank out are

rename (526 bytes)
mkdir (481 bytes)
rmdir (356 bytes)

It may also be possible to bank out some of execve if we don't allow the
code/data of a binary to be the full space so we have room for it.

Other options include just having an overlay zone of about 1.5K which would
hold either

rename
mkdir
rmdir

execve (and helpers)

waitpid
flock