Performance patch for Win32

This is a patch against src/backend/storage/file/fd.c
taken from 9.2beta1.

This patch is submitted for review and comments, not
for application to the code base.  *WIP*

This patch addresses a performance problem stemming
from the use of FindFirstFile() and FindNextFile() to
iterate over a directory in Windows.  These two functions
are used in the port of readdir for Windows.  Unfortunately,
unlike Linux, these Windows directory iteration functions
return the equivalent of a stat() call for each file iterated.
Hence, if a directory contains many tens of thousands of
files, the iteration can take several minutes to complete.

In RemovePgTempFile(), multiple directories are iterated
and all files found which match the pattern for a temporary
file are unlinked.  The pattern matching is performed
*outside* the directory iteration.  This patch uses a file
pattern like "t*" to match all temporary files, rather than
iterating over all files in the directory, thus pushing the
pattern match *inside* the directory iteration and gaining
significant startup time performance.

This is not theoretical.  The real-world database where I
found this problem is on a Windows 2003 server running
PostgreSQL 9.1.3 and having 56,000 tables.  I was able
to duplicate the problem on a Windows 2008 server.

To reproduce, you will need a database on Windows with
tens of thousands of tables and a recent version of
PostgreSQL.  Reboot the Windows server so that the
filesystem is guaranteed not to be in the filesystem cache.
Start postgres using pg_ctl, and note that it takes several
minutes to start.  After applying the patch and re-running
these steps, the server should not take so long to start.

I have the following reservations about my design, and
solicit comments and suggestions for improvement:

1)  The changes I made in fd.c pass a pattern rather than
a name into ReadDir *knowing the details* of how ReadDir
on Windows will use the port of readdir in src/port/dirent.c
and that in that code FindFirstFile() and FindNextFile() will
be called.  This knowledge about the inner workings of
the port of readdir() is not appropriate inside fd.c, IMHO.

2) I used a fair amount of #ifdef WIN32 to avoid adding
unnecessary variables or branches to the non-windows
code.  Since this code is probably not on the critical path
performancewise, this may be overkill.

3) The pattern passed to ReadDir of the form "t*" should
probably be something closer to (in pcre form):
m/^t\d+_\d+/, rather than m/^t.*/.  I am not sufficiently
familiar with how Windows interprets file patterns, and
whether it interprets them differently from one version of
Windows to another, to be comfortable making a more
precise pattern.

4) Other places in the PostgreSQL sources where directory
iteration is needed should probably use a pattern if possible
when running on Windows.  Thus, it might make more
sense to have a version of ReadDir that explicitly takes a
pattern, and use that version of ReadDir elsewhere in the
codebase.