Release version 35

* NEWS: Record release date.
maint: post-release administrivia.

* NEWS: Add header line for next release.
* .prev-version: Record previous version.
* ./local.mk (old_NEWS_hash): Auto-update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
slingshot: use the public slingshot url.

* slingshot: Update.
* bootstrap.slingshot: Sync from upstream.