trs80m1: make trs80-bank more flexible

trs80m1: make trs80-bank more flexible

Rework trs80-bank so that anything that can be represented by

out someport, someconst		-	kernel bank 1
out someport, someconst		-	kernel bank 2

out someport, value		-	user banks

Can be handled with ease.

This code also adds selector support as the second bank type and others can now
be added without much trouble. It's not the whole story as the fork() code
also needs changing and the fast copier is a bit trickier.