Support in-place upgrades of new minio binary and releases. (#4961)

This PR allows 'minio update' to not only shows update banner
but also allows for in-place upgrades.

Updates are done safely by validating the downloaded
sha256 of the binary.

Fixes #4781

vendor/github.com/inconshreveable/go-update/internal/osext/osext_windows.go

@@ -0,0 +1,34 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package osext
+
+import (
+	"syscall"
+	"unicode/utf16"
+	"unsafe"
+)
+
+var (
+	kernel                = syscall.MustLoadDLL("kernel32.dll")
+	getModuleFileNameProc = kernel.MustFindProc("GetModuleFileNameW")
+)
+
+// GetModuleFileName() with hModule = NULL
+func executable() (exePath string, err error) {
+	return getModuleFileName()
+}
+
+func getModuleFileName() (string, error) {
+	var n uint32
+	b := make([]uint16, syscall.MAX_PATH)
+	size := uint32(len(b))
+
+	r0, _, e1 := getModuleFileNameProc.Call(0, uintptr(unsafe.Pointer(&b[0])), uintptr(size))
+	n = uint32(r0)
+	if n == 0 {
+		return "", e1
+	}
+	return string(utf16.Decode(b[0:n])), nil
+}