Replace pgwin32_is_junction() with lstat().

Now that lstat() reports junction points with S_IFLNK/S_ISLINK(), and
unlink() can unlink them, there is no need for conditional code for
Windows in a few places.  That was expressed by testing for WIN32 or
S_ISLNK, which we can now constant-fold.

The coding around pgwin32_is_junction() was a bit suspect anyway, as we
never checked for errors, and we also know that errors can be spuriously
reported because of transient sharing violations on this OS.  The
lstat()-based code has handling for that.

This also reverts 4fc6b6ee on master only.  That was done because
lstat() didn't previously work for symlinks (junction points), but now
it does.

Tested-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/CA%2BhUKGLfOOeyZpm5ByVcAt7x5Pn-%3DxGRNCvgiUPVVzjFLtnY0w%40mail.gmail.com

src/backend/utils/adt/misc.c

@@ -283,9 +283,7 @@ pg_tablespace_location(PG_FUNCTION_ARGS)
 	char		sourcepath[MAXPGPATH];
 	char		targetpath[MAXPGPATH];
 	int			rllen;
-#ifndef WIN32
 	struct stat st;
-#endif
 
 	/*
 	 * It's useful to apply this function to pg_class.reltablespace, wherein
@@ -314,10 +312,6 @@ pg_tablespace_location(PG_FUNCTION_ARGS)
 	 * created with allow_in_place_tablespaces enabled.  If a directory is
 	 * found, a relative path to the data directory is returned.
 	 */
-#ifdef WIN32
-	if (!pgwin32_is_junction(sourcepath))
-		PG_RETURN_TEXT_P(cstring_to_text(sourcepath));
-#else
 	if (lstat(sourcepath, &st) < 0)
 	{
 		ereport(ERROR,
@@ -328,7 +322,6 @@ pg_tablespace_location(PG_FUNCTION_ARGS)
 
 	if (!S_ISLNK(st.st_mode))
 		PG_RETURN_TEXT_P(cstring_to_text(sourcepath));
-#endif
 
 	/*
 	 * In presence of a link or a junction point, return the path pointing to.