pgsql: Simplify uses of readdir() by creating a function ReadDir() that

Log Message:
-----------
Simplify uses of readdir() by creating a function ReadDir() that
includes error checking and an appropriate ereport(ERROR) message.
This gets rid of rather tedious and error-prone manipulation of errno,
as well as a Windows-specific bug workaround, at more than a dozen
call sites. After an idea in a recent patch by Heikki Linnakangas.

Modified Files:
--------------
pgsql/contrib/dbsize:
dbsize.c (r1.17 -> r1.18)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/dbsize/dbsize.c.diff?r1=1.17&r2=1.18)
pgsql/src/backend/access/transam:
slru.c (r1.24 -> r1.25)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/slru.c.diff?r1=1.24&r2=1.25)
twophase.c (r1.4 -> r1.5)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/twophase.c.diff?r1=1.4&r2=1.5)
xlog.c (r1.202 -> r1.203)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.202&r2=1.203)
pgsql/src/backend/commands:
tablespace.c (r1.21 -> r1.22)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablespace.c.diff?r1=1.21&r2=1.22)
pgsql/src/backend/postmaster:
pgarch.c (r1.15 -> r1.16)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgarch.c.diff?r1=1.15&r2=1.16)
pgsql/src/backend/storage/file:
fd.c (r1.116 -> r1.117)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/file/fd.c.diff?r1=1.116&r2=1.117)
pgsql/src/backend/utils/adt:
misc.c (r1.43 -> r1.44)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/misc.c.diff?r1=1.43&r2=1.44)
pgsql/src/include/storage:
fd.h (r1.51 -> r1.52)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/fd.h.diff?r1=1.51&r2=1.52)
pgsql/src/timezone:
pgtz.c (r1.33 -> r1.34)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/timezone/pgtz.c.diff?r1=1.33&r2=1.34)