switch to s6-linux-init and s6-rc

This is at the point where it runs a getty and a pile of s6-supervise
processes, though it doesn't seem to run the things being supervised

overlay.nix

@@ -1,3 +1,5 @@
 final: prev: {
-#   s6-rc = final.callPackage ./pkgs/s6-rc;
+  pseudofile = final.callPackage ./pkgs/pseudofile {};
+  s6-init-files = final.callPackage ./pkgs/s6-init-files {};
+  strace = prev.strace.override { libunwind = null; };
 }