change frequency option for nodes

Updating to the latest 7.x-2.x-dev.

I couldn't easily generate an interdiff because the old patch no longer applied. There were only two main differences:

1) The patch in #79 contained a change from xmlsitemap_get_changefreq_options() to xmlsitemap_get_changefreq_values(), however a few other places in the module were referencing the _options() function, so I excluded that change from the patch.

2) The previous comment had some @file comments which differ from the current branch, and they don't need to be a part of this patch.

Fix to #84 to rename the new options function to xmlsitemap_get_changefreq_form_options().