windows: Support all REPARSE_POINT attrib files properly. (#4203)

This change adopts the upstream fix in this regard at
https://go-review.googlesource.com/#/c/41834/ for Minio's
purposes.

Go's current os.Stat() lacks support for lot of strange
windows files such as

 - share symlinks on SMB2
 - symlinks on docker nanoserver
 - de-duplicated files on NTFS de-duplicated volume.

This PR attempts to incorporate the change mentioned here

   https://blogs.msdn.microsoft.com/oldnewthing/20100212-00/?p=14963/

The article suggests to use Windows I/O manager to
dereference the symbolic link.

Fixes #4122

cmd/posix-utils_nix_test.go

@@ -20,7 +20,6 @@ package cmd
 
 import (
 	"io/ioutil"
-	"os"
 	"path"
 	"syscall"
 	"testing"
@@ -62,7 +61,7 @@ func TestIsValidUmaskVol(t *testing.T) {
 	defer removeAll(tmpPath)
 
 	// Stat to get permissions bits.
-	st, err := os.Stat(path.Join(tmpPath, testCase.volName))
+	st, err := osStat(path.Join(tmpPath, testCase.volName))
 	if err != nil {
 		t.Fatalf("Stat failed with %s expected to pass.", err)
 	}