pgsql: Properly check for readdir/closedir() failures

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.

Branch
------
REL8_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/de13cea4b96d1939703b42cfe96278f3a915ac4e

Modified Files
--------------
contrib/pg_standby/pg_standby.c     |   17 ++++++++--
src/backend/storage/file/fd.c       |    6 +---
src/bin/initdb/initdb.c             |   12 ++-----
src/bin/pg_resetxlog/pg_resetxlog.c |   63 ++++++++++++++++++-----------------
src/port/dirent.c                   |    8 +++--
src/port/dirmod.c                   |   22 +++++++-----
6 files changed, 71 insertions(+), 57 deletions(-)