bootstrap: sync with upstream.

* bootstrap: Pick up some bugfixes and enhancements from earlier
today.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
configury: ChangeLog and README are now managed by bootstrap.

* bootstrap.conf (slingshot_force_changelog)
(slingshot_force_readme): Remove.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>