larch: ignore user LUA_INIT environment variables.

* build-aux/larch (preamble): Reset LUA_INIT and LUA_INIT_5_2
before re-executing with Lua interpreter.
* THANKS: Add Reuben Thomas.
* NEWS: Update.
Reported by Reuben Thomas.

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