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.
compat/common, so that they can be shared by several emulations, and use
them for Darwin.
This removes the ugly dependance on FreeBSD freebsd_file.c for COMPAT_DARWIN