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.go

@@ -0,0 +1,27 @@
+// 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.
+
+// Extensions to the standard "os" package.
+package osext
+
+import "path/filepath"
+
+// Executable returns an absolute path that can be used to
+// re-invoke the current program.
+// It may not be valid after the current program exits.
+func Executable() (string, error) {
+	p, err := executable()
+	return filepath.Clean(p), err
+}
+
+// Returns same path as Executable, returns just the folder
+// path. Excludes the executable name and any trailing slash.
+func ExecutableFolder() (string, error) {
+	p, err := Executable()
+	if err != nil {
+		return "", err
+	}
+
+	return filepath.Dir(p), nil
+}