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/moby/buildkit/util/appcontext/appcontext_windows.go

@@ -0,0 +1,7 @@
+package appcontext
+
+import (
+	"os"
+)
+
+var terminationSignals = []os.Signal{os.Interrupt}