slingshot: sync with upstream, and update copyright years.

* slingshot: Update, particularly for update-copyright support.
* bootstrap, bootstrap.slingshot: Update from slingshot.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
slingshot: sync with upstream and simplify accordingly.

* slingshot: Sync with upstream.
* bootstrap: Update from slingshot.
* configure.ac (AM_INIT_AUTOMAKE): Remove 'foreign'. Slingshot
now handles missing README automatically.
* bootstrap.slingshot: Remove.  No longer required.
* bootstrap.conf: Remove bootstrap.slingshot source boilerplate.
(stdlib_force_changelog): Remove.  Automated by slingshot now.
* INSTALL: Remove.  Autotools copies in the canonical version
automatically.
* COPYING: New file, with MIT License, to avoid Autotools
copying over a standard GPLv3 COPYING boilerplate.
* .gitignore: Update.

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