hurd: Respect existing FD_CLOEXEC in S_msg_set_fd

If the process has set the close-on-exec flag for the file descriptor,
it expects the file descriptor to get closed on exec, even if we replace
what the file descriptor refers to.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230429131354.2507443-6-bugaevc@gmail.com>