Add tests from Linux kernel commit messages for
t/dup3 et al.
Validate completeness according to https://wiki.freebsd.org/AtomicCloseOnExec or a similar list.
In http://lwn.net/Articles/417421/ an interesting point is made: you [may]
want some file descriptor to still be open if 'exec' fails, but you
don't want it to be open after the exec succeeds. I'm not sure
whether our current
O_CLOEXEC implementation adheres to that.