maint: bump release version to 3.

* configure.ac (AC_INIT): Bump release version to 3.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
maint: upgrade to slingshot release files.

* rockspec.conf: lyaml rockspec configuration for slingshot.
* NEWS: New file.  Add release notes from previous versions.
* .prev-version: New file. Required by do-release-commit-and-tag.
* build-aux/do-release-commit-and-tag, build-aux/mkrockspecs,
bulid-aux/release.mk, build-aux/rockspecs.mk: Install from
slingshot upstream.
* build-aux/mkrockspecs.lua: Remove. Obsolete.
* GNUmakefile, Makefile.am: Adjust to work with slingshot files.
* .gitignore, build-aux/.gitignore: Update.
Release version 3

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

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

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