* LICENSE.md, README.md: Add 2018 to copyright years.
* build-aux/config.ld.in, lib/std/_base.lua, lib/std/debug.lua,
lib/std/init.lua, lib/std/io.lua, lib/std/math.lua,
lib/std/package.lua, lib/std/string.lua, lib/std/table.lua,
spec/debug_spec.yaml, spec/io_spec.yaml, spec/math_spec.yaml,
spec/package_spec.yaml, spec/spec_helper.lua, spec/std_spec.yaml,
spec/string_spec.yaml, spec/table_spec.yaml: Likewise.  Remove
individual authors from copyright statements.
* Makefile: Add copyright statement.

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