Add user namespace (mapping) support to the Docker engine

Adds support for the daemon to handle user namespace maps as a
per-daemon setting.

Support for handling uid/gid mapping is added to the builder,
archive/unarchive packages and functions, all graphdrivers (except
Windows), and the test suite is updated to handle user namespace daemon
rootgraph changes.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)

directory/directory_windows.go

@@ -5,7 +5,6 @@ package directory
 import (
 	"os"
 	"path/filepath"
-	"strings"
 
 	"github.com/docker/docker/pkg/longpath"
 )