Remaining issues:

- traps are not handled yet.
- ix_panic() does not work properly (varargs conversion needed).
- 'man' crashes here. I've not investigated why yet.
- programs that rely on low-level internal ixemul structs won't work.
The only one I know doing that is 'gdb'. Recompiling it with some minor
changes should take care of the problem.
- ixtrace is not yet supported.
- neither is profiling.
- not compatible with the powerup bridge currently. That may or may
not change.
- DTYPE_MEM is broken, disabled for now.

The stack size of the 68k emulation is set as usual, with the
'stack' command. The size of the PPC stack is taken from the
IXPPCSTACK environment variable.