[bug-diffutils] bug#33965: handling of closed file descriptors

[Bruno Haible]

I wrote:
> the result of fcntl
> can apparently be used to distinguish the substitute file descriptor
> stuffed in by exec() from a regular open("/dev/null",O_RDONLY).

Note that the code works for STDIN_FILENO, where fcntl(...) is 5 in one case
and 0 in the other case, and we use FD_CLOEXEC = 1 to distinguish the two.
For STDOUT_FILENO, fcntl(...) would be 5 in one case and 1 in the other case,
and the flag which allows to distinguish the two cases (4) is not listed in
<sys/fcntl.h>, therefore likely is a kernel-private flag.

Bruno