readdir is incorrectly implemented at Windows

Hi hackers,

Small issue with readir implementation for Windows.
Right now it returns ENOENT in case of any error returned by FindFirstFile.
So all places in Postgres where opendir/listdir are used will assume 
that directory is empty and
do nothing without reporting any error.
It is not so good if directory is actually not empty but there are not 
enough permissions for accessing the directory and FindFirstFile
returns ERROR_ACCESS_DENIED:

struct dirent *
readdir(DIR *d)
{
     WIN32_FIND_DATA fd;

     if (d->handle == INVALID_HANDLE_VALUE)
     {
         d->handle = FindFirstFile(d->dirname, &fd);
         if (d->handle == INVALID_HANDLE_VALUE)
         {
             errno = ENOENT;
             return NULL;
         }
     }


Attached please find small patch fixing the problem.

-- 
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company