Support priv/unpriv image extraction

Optionally add back privileged extraction which can be enabled with
LUET_PRIVILEGED_EXTRACT=true

Signed-off-by: Ettore Di Giacinto <mudler@sabayon.org>

vendor/github.com/genuinetools/img/AUTHORS

@@ -0,0 +1,16 @@
+# This file lists all individuals having contributed content to the repository.
+# For how it is generated, see `make AUTHORS`.
+
+Akihiro Suda <suda.kyoto@gmail.com> 
+Anand Patil <anand.prabhakar.patil@gmail.com> 
+Daniel Schep <dschep@gmail.com> 
+David Medberry <openstack@medberry.net> 
+Derek McGowan <derek@mcgstyle.net> 
+Eric Hotinger <ehotinger@gmail.com> 
+hansmi <hansmi@users.noreply.github.com> 
+Jessica Frazelle <acidburn@microsoft.com> 
+Matthew Fisher <matt.fisher@microsoft.com> 
+nogoegst <nogoegst@users.noreply.github.com> 
+Paul Tagliamonte <paultag@gmail.com> 
+Ryan Cox <ryancox@users.noreply.github.com> 
+Ryan Dunckel <sparty02@gmail.com>

vendor/github.com/genuinetools/img/LICENSE

@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (c) 2018 The Genuinetools Authors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

vendor/github.com/genuinetools/img/types/types.go

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+package types
+
+const (
+	// AutoBackend is automatically resolved into either overlayfs or native.
+	AutoBackend = "auto"
+	// NativeBackend defines the native backend.
+	NativeBackend = "native"
+	// OverlayFSBackend defines the overlayfs backend.
+	OverlayFSBackend = "overlayfs"
+)