Use admin_url() to generate settings form action

If WordPress core files are in a subdirectory (eg. in https://github.com/markjaquith/WordPress-Skeleton), `$_SERVER['PHP_SELF']` returns the rewritten URL path. This causes the user to be redirected and logged out of the admin area when updating the settings. Using the built-in `admin_url()` function seems to give a more consistent result.