Properly check for readdir/closedir() failures

Bruce Momjian <bruce@momjian.us>

Commit: 6f03927fce038096f53ca67eeab9adb24938f8a6
Author: Bruce Momjian <bruce@momjian.us>
Date: 2014-03-21T17:45:11Z
Releases: 9.4.0
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