lighttpd: add gitweb as a sub-service

Now you can access gitweb at http://yourserver/gitweb by simply adding
this to configuration.nix:

  services.lighttpd.gitweb.enable = true;

The path to all bare repositories served by gitweb can be set with this
option (default value below):

  services.lighttpd.gitweb.projectroot = "/srv/git";