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_procfs.go

@@ -0,0 +1,36 @@
+// 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.
+
+// +build linux netbsd openbsd solaris dragonfly
+
+package osext
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"runtime"
+	"strings"
+)
+
+func executable() (string, error) {
+	switch runtime.GOOS {
+	case "linux":
+		const deletedTag = " (deleted)"
+		execpath, err := os.Readlink("/proc/self/exe")
+		if err != nil {
+			return execpath, err
+		}
+		execpath = strings.TrimSuffix(execpath, deletedTag)
+		execpath = strings.TrimPrefix(execpath, deletedTag)
+		return execpath, nil
+	case "netbsd":
+		return os.Readlink("/proc/curproc/exe")
+	case "openbsd", "dragonfly":
+		return os.Readlink("/proc/curproc/file")
+	case "solaris":
+		return os.Readlink(fmt.Sprintf("/proc/%d/path/a.out", os.Getpid()))
+	}
+	return "", errors.New("ExecPath not implemented for " + runtime.GOOS)
+}