nixos/tcsd: several improvements and fixes

- Actually run tcsd as tss/tss
- Install a udev rule to set /dev/tpm* permissions
- Remove systemd-udev-settle dependency, use dev-tpm0.device instead
- Use systemd-tmpfiles to set up the state directory
- Add documentation URI to tcsd.service

This module cannot be easily tested with a NixOS test due to the TPM
dependency. Technically, one could be emulated using swtpm[1], but this
is not packaged in Nixpkgs. If you computer has a real TPM you can do a
passthrough in Qemu, but this requires running the VM as root and of
course it's not determinstic:

    $ nix build -f nixos vm --arg configuration '
      {
        virtualisation.qemu.options = [
          "-tpmdev passthrough,id=tpm0,path=/dev/tpm0,cancel-path=/sys/class/tpm/tpm0/cancel"
          "-device tpm-tis,tpmdev=tpm0"
        ];
        users.users.root.hashedPassword = "";
        services.tcsd.enable = true;
      }'

After starting the VM, log in as root, you can check the service has
started with `systemctl status tcsd`.

[1]: https://github.com/stefanberger/swtpm

nixos/modules/services/hardware/tcsd.nix

@@ -119,22 +119,31 @@ in
 
     environment.systemPackages = [ pkgs.trousers ];
 
-#    system.activationScripts.tcsd =
-#      ''
-#        chown ${cfg.user}:${cfg.group} ${tcsdConf}
-#      '';
+    services.udev.extraRules = ''
+      # Give tcsd ownership of all TPM devices
+      KERNEL=="tpm[0-9]*", MODE="0660", OWNER="${cfg.user}", GROUP="${cfg.group}"
+      # Tag TPM devices to create a .device unit for tcsd to depend on
+      ACTION=="add", KERNEL=="tpm[0-9]*", TAG+="systemd"
+    '';
+
+    systemd.tmpfiles.rules = [
+      # Initialise the state directory
+      "d ${cfg.stateDir} 0770 ${cfg.user} ${cfg.group} - -"
+    ];
 
     systemd.services.tcsd = {
-      description = "TCSD";
-      after = [ "systemd-udev-settle.service" ];
+      description = "Manager for Trusted Computing resources";
+      documentation = [ "man:tcsd(8)" ];
+
+      requires = [ "dev-tpm0.device" ];
+      after = [ "dev-tpm0.device" ];
       wantedBy = [ "multi-user.target" ];
-      path = [ pkgs.trousers ];
-      preStart =
-        ''
-        mkdir -m 0700 -p ${cfg.stateDir}
-        chown -R ${cfg.user}:${cfg.group} ${cfg.stateDir}
-        '';
-      serviceConfig.ExecStart = "${pkgs.trousers}/sbin/tcsd -f -c ${tcsdConf}";
+
+      serviceConfig = {
+        User = cfg.user;
+        Group = cfg.group;
+        ExecStart = "${pkgs.trousers}/sbin/tcsd -f -c ${tcsdConf}";
+      };
     };
 
     users.users = optionalAttrs (cfg.user == "tss") {