User state settings:
- Allow empty cookie value to be saved.
- Use get_current_user_id() instead of wp_get_current_user().
- Use is_user_member_of_blog().
- Sanitize both name and value in wp_set_all_user_settings().
- Some var names and code formatting cleanup.
Fixes #24921.
Built from https://develop.svn.wordpress.org/trunk@25109


git-svn-id: https://core.svn.wordpress.org/trunk@25091 1a063a9b-81f0-0310-95a4-ce76da25c4cd