68000: clean up most of the 16bitisms and sign stuff

68000: clean up most of the 16bitisms and sign stuff

Mostly this just tidies up all the introduced types. Various things are made
explicitly unsigned that were not called out before and which gcc rightly warned
on.

The 32bit execve needs are quite different, so we split into two execve() designs
rather than make a mess of the already quite nasty 8bit one.