While we are here, try to tag machine dependent functions in header files.
also transformed darwin_ppc_*_state into mach_ppc_*_state, as this is
what they really are (COMPAT_DARWIN is on the top of COMPAT_MACH, not the
other way around)
two ways:
- the child gets its pid as retval[0] (userland stub will turn it into a 0),
retval[1] is 1 and it is 0 in the parent.
- in the child, the fork syscall is successful, hence we must skip the next
instruction.