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/utils.go

@@ -25,7 +25,6 @@ import (
 	"io"
 	"net/http"
 	"net/url"
-	"os"
 	"strings"
 	"time"
 
@@ -182,7 +181,7 @@ func dumpRequest(r *http.Request) string {
 
 // isFile - returns whether given path is a file or not.
 func isFile(path string) bool {
-	if fi, err := os.Stat(path); err == nil {
+	if fi, err := osStat(path); err == nil {
 		return fi.Mode().IsRegular()
 	}