larch: ignore user LUA_INIT environment variables.

* lib/larch/main.lua (preamble): Reset LUA_INIT and LUA_INIT_5_2
before re-executing with Lua interpreter.
* bin/larch: Regenerate.
* THANKS: Add Reuben Thomas.
* NEWS: Update.
Reported by Reuben Thomas.

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

* slingshot: Sync with upstream for grep GNUism fix.
* bootstrap: Update from slingshot.
* bootstrap.conf (bulidreq): Don't set a git version number, or
else `GIT=true ./bootstrap` doesn't work!
* .travis.yml: Regenerate.

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