convert mount service to trigger

Good: this means it's not hanging holding the s6 dataase lock.

Bad: it's the ugliest implementation and doesn't deserve to be preserved

(tbf the ugliness is not new)

tests/inout/test.nix

@@ -0,0 +1,26 @@
+{
+  liminix
+, nixpkgs
+}:
+let img = (import liminix {
+      device = import "${liminix}/devices/qemu/";
+      liminix-config = ./configuration.nix;
+    }).outputs.vmroot;
+    pkgs = import <nixpkgs> { overlays = [(import ../../overlay.nix)]; };
+in pkgs.runCommand "check" {
+  nativeBuildInputs = with pkgs; [
+    expect
+    socat
+    e2fsprogs
+    util-linux # for sfdisk, fallocate
+  ] ;
+} ''
+mkdir vm
+dd if=/dev/zero of=./vm/stick.e2fs bs=1M count=32
+mkfs.ext2 -L backup-disk ./vm/stick.e2fs
+cat <(dd if=/dev/zero bs=512 count=4)  ./vm/stick.e2fs > ./vm/stick.img
+echo '4,-,L,*' | sfdisk ./vm/stick.img
+
+${img}/run.sh  --background ./vm --flag -device --flag usb-ehci,id=xhci --flag -drive  --flag if=none,id=usbstick,format=raw,file=$(pwd)/vm/stick.img
+expect ${./script.expect} | tee $out
+''