openat: avoid using wrong fd

openat: avoid using wrong fd

Detected by a Solaris failure on:
int fd = dup (0);
close (fd);
mkdirat (fd, "dir", 0700);
which created "./dir" instead of failing with EBADF.

* lib/openat.c (openat_permissive): Reject user's fd if saving the
working directory chooses same fd.
* lib/at-func.c (AT_FUNC_NAME): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>