Software reset - extend supervisor stack.

The value in D1.L for a software reset (CALL 390,<reset data>) now uses bits 5
and 6 of the value passed in in D0.L to extend the size of the supervisor
stack.

The value, 0..3, in those two bits will add that multiple of 8K bytes to the
supervisor stack. I.e. by default, just the normal stack size is allocated. By
setting the bits, 8K, 16K or 24K may be added.

This modification was prompted by people who wished to use C code inside
device drivers, when the existing, rather minimal, supervisor stack was
proving very inadequate.