fix pg_mkdir_p to tolerate concurrent directory creation

While working on the pytest stuff, I found this issue when making it
work on Windows, but the issue can exist everywhere. pg_mkdir_p can fail
if there is a concurrent directory creation. This patch makes it
tolerant of concurrent directory creation. To make it more robust it
stops using stat() on Windows and instead uses GetFileAttributes().

I think this makes the function actually meet the contract set out in
the file's header comment:

 * pg_mkdir_p --- create a directory and, if necessary, parent directories
 *
 * This is equivalent to "mkdir -p" except we don't complain if the target
 * directory already exists.

Thanks to Bryan Green for help with the Windows part.

There might be a few call sites that actually work around this bug that
we can clean up.

cheers

andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com