Windows: Add volume support

Signed-off-by: John Howard <jhoward@microsoft.com>

system/syscall_unix.go

@@ -0,0 +1,11 @@
+// +build linux freebsd
+
+package system
+
+import "syscall"
+
+// UnmountWithSyscall is a platform-specific helper function to call
+// the unmount syscall.
+func UnmountWithSyscall(dest string) {
+	syscall.Unmount(dest, 0)
+}

system/syscall_windows.go

@@ -0,0 +1,6 @@
+package system
+
+// UnmountWithSyscall is a platform-specific helper function to call
+// the unmount syscall. Not supported on Windows
+func UnmountWithSyscall(dest string) {
+}