pam_exec: fix the stderr close test and avoid a double close

[ojanerif_amd.com]

Summary

The cleanup tested cherr[0] when closing cherr[1]. The parent also closed
chin[0]/chout[1]/cherr[1] after pdfork() without clearing them, so the out:
cleanup path closed them again. Clear each fd after closing and test the right
one.

MFC after: 1 week

[markj]

the error path closed them again

It's not an error path as far as I can see.

[ojanerif_amd.com]

Right, it's reached on success too -- reworded to "the out: cleanup path".

[markj]

I'm not sure how you are uploading the patches, but something's a bit off, there's a b/ prefix in the file path. https://wiki.freebsd.org/Phabricator has some notes on how to upload patch files.

[ojanerif_amd.com]

Thanks, you're right. I generated these with diff(1) over two trees (a/ and b/)
rather than git, so the headers are "--- a/<path>" / "+++ b/<path>" with no
"diff --git" line. I'll regenerate diffs with git diff -U999999 from now on so
the paths come out clean. Thanks for the link.