Release version 5.1.0

* 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>
configury: bump release number to 5.1.1.

* configure.ac (AC_INIT): Bump release number to 5.1.1.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
slingshot: sync with upstream, for luaexec_LTLIBRARIES support.

Close #5.
* slingshot: Sync with upstream.
* bootstrap: Update from slingshot.
* .travis.yml: Regenerate.
* local.mk (lib_LTLIBRARIES): Rename from this...
(luaexec_LTLIBRARIES): ...to this.
(old_NEWS_hash): Update.
* NEWS: Move from here...
* NEWS.md: ...to here.  Reformat and update.
* .gitignore: Add NEWS.

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