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.go

@@ -1,4 +1,4 @@
-// +build linux darwin dragonfly freebsd netbsd openbsd solaris
+// +build !windows
 
 /*
  * Minio Cloud Storage, (C) 2016, 2017 Minio, Inc.
@@ -20,6 +20,12 @@ package cmd
 
 import "os"
 
+// osStat returns a FileInfo structure describing the named file.
+// If there is an error, it will be of type *PathError.
+func osStat(name string) (os.FileInfo, error) {
+	return os.Stat(name)
+}
+
 // isValidVolname verifies a volname name in accordance with object
 // layer requirements.
 func isValidVolname(volname string) bool {