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

tests/pseudofiles/run.sh

@@ -0,0 +1,17 @@
+set -e
+
+expr=$(cat <<"EXPR"
+let
+  overlay = import ./overlay.nix;
+  nixpkgs = import <nixpkgs> (  {overlays = [overlay]; });
+  structure = import ./tests/pseudofiles/structure.nix;
+in nixpkgs.pkgs.pseudofile "pseudo.s6-init" structure
+EXPR
+       )
+
+NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1 nix-build -E "${expr}" -o tests/pseudofiles/result $*
+diff tests/pseudofiles/result tests/pseudofiles/result.expected
+test -f  /tmp/out.squashfs && rm /tmp/out.squashfs
+nix-shell -p squashfsTools -p qprint --run "mksquashfs - /tmp/out.squashfs -p '/ d 755 0 0' -pf tests/pseudofiles/result -quiet -no-progress"
+foo="$(nix-shell -p squashfsTools --run 'unsquashfs -cat /tmp/out.squashfs service/s6-linux-init-runleveld/run')"
+test "$foo" = "$(printf "hello\nworld")"