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/segmentio/go-prompt/prompt.go

@@ -0,0 +1,94 @@
+package prompt
+
+import "github.com/howeyc/gopass"
+import "strings"
+import "strconv"
+import "fmt"
+
+// String prompt.
+func String(prompt string, args ...interface{}) string {
+	var s string
+	fmt.Printf(prompt+": ", args...)
+	fmt.Scanln(&s)
+	return s
+}
+
+// String prompt (required).
+func StringRequired(prompt string, args ...interface{}) (s string) {
+	for strings.Trim(s, " ") == "" {
+		s = String(prompt, args...)
+	}
+	return s
+}
+
+// Confirm continues prompting until the input is boolean-ish.
+func Confirm(prompt string, args ...interface{}) bool {
+	for {
+		switch String(prompt, args...) {
+		case "Yes", "yes", "y", "Y":
+			return true
+		case "No", "no", "n", "N":
+			return false
+		}
+	}
+}
+
+// Choose prompts for a single selection from `list`, returning in the index.
+func Choose(prompt string, list []string) int {
+	fmt.Println()
+	for i, val := range list {
+		fmt.Printf("  %d) %s\n", i+1, val)
+	}
+
+	fmt.Println()
+	i := -1
+
+	for {
+		s := String(prompt)
+
+		// index
+		n, err := strconv.Atoi(s)
+		if err == nil {
+			if n > 0 && n <= len(list) {
+				i = n - 1
+				break
+			} else {
+				continue
+			}
+		}
+
+		// value
+		i = indexOf(s, list)
+		if i != -1 {
+			break
+		}
+	}
+
+	return i
+}
+
+// Password prompt.
+func Password(prompt string, args ...interface{}) string {
+	fmt.Printf(prompt+": ", args...)
+	password, _ := gopass.GetPasswd()
+	s := string(password[0:])
+	return s
+}
+
+// Password prompt with mask.
+func PasswordMasked(prompt string, args ...interface{}) string {
+	fmt.Printf(prompt+": ", args...)
+	password, _ := gopass.GetPasswdMasked()
+	s := string(password[0:])
+	return s
+}
+
+// index of `s` in `list`.
+func indexOf(s string, list []string) int {
+	for i, val := range list {
+		if val == s {
+			return i
+		}
+	}
+	return -1
+}