In contrast to rename default behaviour, Linux' renameat2 returns -1
with errno set to EEXIST, if oldfile and newfile refer to the same
file, and the RENAME_NOREPLACE flag is set.
Follow suit, given this is a Linux-only function anyway.
Fixes: f665b1cef30f ("cygwin: Implement renameat2")
Reported-by: Bruno Haible <bruno@clisp.org>
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>