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
------
REL9_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/d73cc5857faca215ee95c858e836bcc12d1d1b70

Modified Files
--------------
contrib/pg_archivecleanup/pg_archivecleanup.c |   16 ++++++-
contrib/pg_standby/pg_standby.c               |   17 ++++++-
contrib/pg_upgrade/file.c                     |   14 +++++-
src/backend/storage/file/fd.c                 |    6 +--
src/bin/pg_resetxlog/pg_resetxlog.c           |   63 +++++++++++++------------
src/port/dirent.c                             |    8 +++-
src/port/dirmod.c                             |   22 +++++----
src/port/pgcheckdir.c                         |   14 ++----
8 files changed, 97 insertions(+), 63 deletions(-)