tailscale: add `interfaceName` option

tailscale allows to specify the interface name.
The upstream systemd unit does not expose it directly however, only
via the `FLAGS` environment variable.

I can’t be 100% sure that the escaping is correct, but this is as good
as we can do for now, unless upstream changes their unit file.

nixos/modules/services/networking/tailscale.nix

···
       description = "The port to listen on for tunnel traffic (0=autoselect).";
     };
 
+    interfaceName = mkOption {
+      type = types.str;
+      default = "tailscale0";
+      description = ''The interface name for tunnel traffic. Use "userspace-networking" (beta) to not use TUN.'';
+    };
+
     package = mkOption {
       type = types.package;
       default = pkgs.tailscale;
···
     systemd.services.tailscaled = {
       wantedBy = [ "multi-user.target" ];
       path = [ pkgs.openresolv ];
-      serviceConfig.Environment = "PORT=${toString cfg.port}";
+      serviceConfig.Environment = [
+        "PORT=${toString cfg.port}"
+        ''"FLAGS=--tun ${lib.escapeShellArg cfg.interfaceName}"''
+      ];
     };
   };
 }