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/config-migrate_test.go

@@ -45,7 +45,7 @@ func TestServerConfigMigrateV1(t *testing.T) {
 		t.Fatal("Unexpected error: ", err)
 	}
 	// Check if config v1 is removed from filesystem
-	if _, err := os.Stat(configPath); err == nil || !os.IsNotExist(err) {
+	if _, err := osStat(configPath); err == nil || !os.IsNotExist(err) {
 		t.Fatal("Config V1 file is not purged")
 	}