z80: introduce a common z80fixedbank.s

This is sufficient for most setups using fixed banks and a common.
mtx; switch to the shared z80 banking code
trs80: move to E800

Make room make room
trs80: use shared bank code
z80pack: Switch to shared z80 banking code
z80pack: add the map_process_a method needed by shared bank code
trs80, fd: update the floppy disk driver

This is sufficient to mount the root file system and go hunting for init. It
will probably go further still once the other bits are in place
trs80: 80x24 not 80x25
trs80: Add serial, keyboard and interrupt handling

This gives a console you can type into and which is at least good enough
to set the root fs. It follows the standard TRS80 layout but with some extra
ctrl and ctrl-shift hooks for useful keys Tandy forgot to include ({} etc)
zx128: We can't yet use the shared banker for this, but prepare anyway