rm: be even more careful when using a replacement errno value

rm: be even more careful when using a replacement errno value

* src/remove.c (excise): The change in commit v8.19-107-gccbd3f3 made
the "rm -rf D" (for unreadable dir, D) diagnostic worse on Solaris 10:
  -rm: cannot remove 'D': Permission denied
  +rm: cannot remove 'D': File exists
That happened because unlinkat would fail with EEXIST there, given
an unreadable directory, which made the two tests, tests/rm/unread2
and tests/rm/unreadable fail.  Accommodate the EEXIST case, too.