rm: be more careful when using a replacement errno value

rm: be more careful when using a replacement errno value

* src/remove.c (excise): Tighten the test for when we defer to an
old errno value: instead of relying solely on an FTS_DNR (unreadable
directory) failure, also test current and replacement errno values.
This change would also have solved the problem addressed by commit
v8.19-106-g57dd067.  For more info, see http://bugs.gnu.org/12339#113