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_plan9.go

@@ -0,0 +1,20 @@
+// 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 (
+	"os"
+	"strconv"
+	"syscall"
+)
+
+func executable() (string, error) {
+	f, err := os.Open("/proc/" + strconv.Itoa(os.Getpid()) + "/text")
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+	return syscall.Fd2path(int(f.Fd()))
+}