Make pg_mkdir_p() tolerant of a concurrent directory creation.

Make pg_mkdir_p() tolerant of a concurrent directory creation.

pg_mkdir_p creates each missing path component with a stat() followed
by mkdir().  If the stat() reports the component as absent but another
process creates it in the window before this process's mkdir(), mkdir()
fails with EEXIST and pg_mkdir_p treated that as a hard error -- unlike
"mkdir -p", which is meant to be idempotent and race-tolerant.

This shows up when several processes concurrently create paths that
share an ancestor directory: for example, parallel initdb runs whose
data directories live under a common temporary directory.  One process
wins the race to create the shared ancestor and the others fail with
    could not create directory "...": File exists

Fix this race condition by first trying mkdir() and only attempting
stat() if it fails with EEXIST.

On Windows, there's an additional problem: stat() opens a file handle
and participates in share-mode locking, which means it can transiently
fail on a directory another process is concurrently creating.  Use
GetFileAttributes() instead: it requests only FILE_READ_ATTRIBUTES
and is exempt from share-mode denial, so it reliably sees a
concurrently-created directory.

I (tgl) also chose to back-patch 039f7ee0f's effects on this function,
so that pgmkdirp.c remains identical in all live branches.

Author: Andrew Dunstan <andrew@dunslane.net>
Co-authored-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3ca004de-e49b-4471-b8aa-fd656e70f68c@dunslane.net
Backpatch-through: 14