Support detecting mountpoints correctly (#7288)

Currently windows support was relying on Symlink as
a way to detect a drive, this doesn't work in latest
Windows 2016, fix this to use a proper mechanism by
using win32 APIs.

Additionally also add support for detecting bind mounts
on Linux.

vendor/golang.org/x/sys/windows/str.go

@@ -0,0 +1,22 @@
+// Copyright 2009 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 windows
+
+package windows
+
+func itoa(val int) string { // do it here rather than with fmt to avoid dependency
+	if val < 0 {
+		return "-" + itoa(-val)
+	}
+	var buf [32]byte // big enough for int64
+	i := len(buf) - 1
+	for val >= 10 {
+		buf[i] = byte(val%10 + '0')
+		i--
+		val /= 10
+	}
+	buf[i] = byte(val + '0')
+	return string(buf[i:])
+}