Expand description
dup(fd)—Creates a new OwnedFd instance that shares the same
underlying file description as fd.
This function does not set the O_CLOEXEC flag. To do a dup that does
set O_CLOEXEC, use fcntl_dupfd_cloexec.
POSIX guarantees that dup will use the lowest unused file descriptor,
however it is not safe in general to rely on this, as file descriptors may
be unexpectedly allocated on other threads or in libraries.