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/howeyc/gopass/README.md

@@ -0,0 +1,27 @@
+# getpasswd in Go [![GoDoc](https://godoc.org/github.com/howeyc/gopass?status.svg)](https://godoc.org/github.com/howeyc/gopass) [![Build Status](https://secure.travis-ci.org/howeyc/gopass.png?branch=master)](http://travis-ci.org/howeyc/gopass)
+
+Retrieve password from user terminal or piped input without echo.
+
+Verified on BSD, Linux, and Windows.
+
+Example:
+```go
+package main
+
+import "fmt"
+import "github.com/howeyc/gopass"
+
+func main() {
+	fmt.Printf("Password: ")
+
+	// Silent. For printing *'s use gopass.GetPasswdMasked()
+	pass, err := gopass.GetPasswd()
+	if err != nil {
+		// Handle gopass.ErrInterrupted or getch() read error
+	}
+
+	// Do something with pass
+}
+```
+
+Caution: Multi-byte characters not supported!