Properly check for readdir/closedir() failures

Bruce Momjian <bruce@momjian.us>

Commit: 4859a710ea9b26e38a9637301f1259f6123e4de7
Author: Bruce Momjian <bruce@momjian.us>
Date: 2014-03-21T17:45:11Z
Releases: 9.0.18
Properly check for readdir/closedir() failures

Clear errno before calling readdir() and handle old MinGW errno bug
while adding full test coverage for readdir/closedir failures.

Backpatch through 8.4.

Files