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/binarydist/seek.go

@@ -0,0 +1,43 @@
+package binarydist
+
+import (
+	"errors"
+)
+
+type seekBuffer struct {
+	buf []byte
+	pos int
+}
+
+func (b *seekBuffer) Write(p []byte) (n int, err error) {
+	n = copy(b.buf[b.pos:], p)
+	if n == len(p) {
+		b.pos += n
+		return n, nil
+	}
+	b.buf = append(b.buf, p[n:]...)
+	b.pos += len(p)
+	return len(p), nil
+}
+
+func (b *seekBuffer) Seek(offset int64, whence int) (ret int64, err error) {
+	var abs int64
+	switch whence {
+	case 0:
+		abs = offset
+	case 1:
+		abs = int64(b.pos) + offset
+	case 2:
+		abs = int64(len(b.buf)) + offset
+	default:
+		return 0, errors.New("binarydist: invalid whence")
+	}
+	if abs < 0 {
+		return 0, errors.New("binarydist: negative position")
+	}
+	if abs >= 1<<31 {
+		return 0, errors.New("binarydist: position out of range")
+	}
+	b.pos = int(abs)
+	return abs, nil
+}