nginx module: add option to make vhost default

nixos/modules/services/web-servers/nginx/default.nix

···
       let
         ssl = vhost.enableSSL || vhost.forceSSL;
         port = if vhost.port != null then vhost.port else (if ssl then 443 else 80);
-        listenString = toString port + optionalString ssl " ssl spdy";
+        listenString = toString port + optionalString ssl " ssl spdy"
+          + optionalString vhost.default " default";
         acmeLocation = optionalString vhost.enableACME ''
           location /.well-known/acme-challenge {
             try_files $uri @acme-fallback;
···
       in ''
         ${optionalString vhost.forceSSL ''
           server {
-            listen 80;
-            listen [::]:80;
+            listen 80 ${optionalString vhost.default "default"};
+            listen [::]:80 ${optionalString vhost.default "default"};
 
             server_name ${serverName} ${concatStringsSep " " vhost.serverAliases};
             ${acmeLocation}

nixos/modules/services/web-servers/nginx/vhost-options.nix

···
       '';
     };
 
+    default = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Makes this vhost the default.
+      '';
+    };
+
     extraConfig = mkOption {
       type = types.lines;
       default = "";