Autosave: prevent setting multiple stale `wp-saving-post-*` cookies when the browser disregards "session cookies" expiration on quit:
- Add expiration time of 24 hours for these cookies.
- Rename them to `wp-saving-post` (no post_id) so there is never more than one cookie per domain.
Fixes #29294.
Built from https://develop.svn.wordpress.org/trunk@29572


git-svn-id: http://core.svn.wordpress.org/trunk@29346 1a063a9b-81f0-0310-95a4-ce76da25c4cd