bug#81377: mv -Tn suceeds if target directory exists, no error printed

* Florian Weimer via:

> Based on the documentation, I would have expected an error in this case.
>
> mkdir a a/1 b b/2
> mv -Tn a b
>
> Exit status is zero.  The only relevant system call in the strace output
> is this one:
>
> renameat2(AT_FDCWD, "a", AT_FDCWD, "b", RENAME_NOREPLACE) =3D -1 EEXIST (=
File exists)
>
> Seen with: coreutils-9.7-9.fc43.x86_64

I see that there is a hint in the -n description that the exit status
may not be what users expects.  But it means that this claim in the
manual is slightly misleading:

| However, if =E2=80=98mv -T /tmp/source /tmp/dest=E2=80=99 succeeds, there=
 is no
| question that =E2=80=98/tmp/source=E2=80=99 was renamed to =E2=80=98/tmp/=
dest=E2=80=99.

Florian