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/include/port/win32_port.h

@@ -230,7 +230,6 @@ int			setitimer(int which, const struct itimerval *value, struct itimerval *oval
  */
 extern int	pgsymlink(const char *oldpath, const char *newpath);
 extern int	pgreadlink(const char *path, char *buf, size_t size);
-extern bool pgwin32_is_junction(const char *path);
 
 #define symlink(oldpath, newpath)	pgsymlink(oldpath, newpath)
 #define readlink(path, buf, size)	pgreadlink(path, buf, size)