Function close_fds::set_fds_cloexec

pub fn set_fds_cloexec(minfd: c_int, keep_fds: &[c_int])

Identical to close_open_fds(), but sets the FD_CLOEXEC flag on the file descriptors instead of closing them.

On some platforms (most notably, some of the BSDs), this is significantly less efficient than close_open_fds(), and use of that function should be preferred when possible.