Set sitemap default to 0.5

As suggested by John Kenney in http://drupal.org/node/157533#comment-821430 the default for sitemaps should be 0.5 rather than 0.1.

He comments:

Why is the default set to 0.1? sitemaps.org indicates the normal default is 0.5. why not use that? having it set to near lowest level means you have to change practically every page - or at least i did. it is good that you can change in the admin panel, but that only affects new pages.