Windows: Factor down criu_opts

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

libcontainer/criu_opts_unix.go

@@ -1,3 +1,5 @@
+// +build linux freebsd
+
 package libcontainer
 
 // cgroup restoring strategy provided by criu

libcontainer/criu_opts_windows.go

@@ -0,0 +1,6 @@
+package libcontainer
+
+// TODO Windows: This can ultimately be entirely factored out as criu is
+// a Unix concept not relevant on Windows.
+type CriuOpts struct {
+}