r343896 made it such that a non-zero exit status was passed through, but was
still wrong if the script exits on a signal. POSIX does not say what the
WEXITSTATUS macro returns in this case and in practice 0 is a common value.
Instead, translate the wait status into 8 bits the same way as the shell
calculates $?.
Reviewed by: kib, Nash Kaminski
MFC after: 1 week
3b08e0fcf3