u9fs service: switch user with systemd

Drop the broken fsRoot option.

nixos/modules/services/network-filesystems/u9fs.nix

@@ -27,10 +27,17 @@ in
         '';
       };
 
+      user = mkOption {
+        type = types.str;
+        default = "nobody";
+        description =
+          "User to run u9fs under.";
+      };
+
       extraArgs = mkOption {
         type = types.str;
         default = "";
-        example = "-a none -u nobody";
+        example = "-a none";
         description =
           ''
             Extra arguments to pass on invocation,
@@ -38,13 +45,6 @@ in
           '';
       };
 
-      fsroot = mkOption {
-        type = types.path;
-        default = "/";
-        example = "/srv";
-        description = "File system root to serve to clients.";
-      };
-
     };
 
   };
@@ -63,9 +63,10 @@ in
         reloadIfChanged = true;
         requires = [ "u9fs.socket" ];
         serviceConfig =
-          { ExecStart = "-${pkgs.u9fs}/bin/u9fs ${cfg.extraArgs} ${cfg.fsroot}";
+          { ExecStart = "-${pkgs.u9fs}/bin/u9fs ${cfg.extraArgs}";
             StandardInput = "socket";
             StandardError = "journal";
+            User = cfg.user;
           };
       };
     };