Fix readlink() for non-PostgreSQL junction points on Windows.

Since commit c5cb8f3b taught stat() to follow symlinks, and since initdb
uses pg_mkdir_p(), and that examines parent directories, our humble
readlink() implementation can now be exposed to junction points not of
PostgreSQL origin.  Those might be corrupted by our naive path mangling,
which doesn't really understand NT paths in general.

Simply decline to transform paths that don't look like a drive absolute
path.  That means that readlink() returns the NT path directly when
checking a parent directory of PGDATA that happen to point to a drive
using "rooted" format.  That  works for the purposes of our stat()
emulation.

Reported-by: Roman Zharkov <r.zharkov@postgrespro.ru>
Reviewed-by: Roman Zharkov <r.zharkov@postgrespro.ru>
Discussion: https://postgr.es/m/4590c37927d7b8ee84f9855d83229018%40postgrespro.ru
Discussion: https://postgr.es/m/CA%2BhUKG%2BajSQ_8eu2AogTncOnZ5me2D-Cn66iN_-wZnRjLN%2Bicg%40mail.gmail.com

Backpatched commit f71007fb as above by Thomas Munro into releases 13 thru 15

Discussion: https://postgr.es/m/CA+hUKGLbnv+pe3q1fYOVkLD3pMra7GuihfMxUN-1831YH9RYQg@mail.gmail.com

src/port/dirmod.c

@@ -351,10 +351,21 @@ pgreadlink(const char *path, char *buf, size_t size)
 	}
 
 	/*
-	 * If the path starts with "\??\", which it will do in most (all?) cases,
+	 * If the path starts with "\??\" followed by a "drive absolute" path
-	 * strip those out.
+	 * (known to Windows APIs as RtlPathTypeDriveAbsolute), then strip that
+	 * prefix.  This undoes some of the transformation performed by
+	 * pqsymlink(), to get back to a format that users are used to seeing.  We
+	 * don't know how to transform other path types that might be encountered
+	 * outside PGDATA, so we just return them directly.
 	 */
-	if (r > 4 && strncmp(buf, "\\??\\", 4) == 0)
+	if (r >= 7 &&
+		buf[0] == '\\' &&
+		buf[1] == '?' &&
+		buf[2] == '?' &&
+		buf[3] == '\\' &&
+		isalpha(buf[4]) &&
+		buf[5] == ':' &&
+		buf[6] == '\\')
 	{
 		memmove(buf, buf + 4, strlen(buf + 4) + 1);
 		r -= 4;