Remove obsolete comment.
Get rid of the global traceback index.

Just push the traceback function before lua_pcall. The now has to be
less meticulously maintained between lua_setup and lua_run.
Get rid of static lua-exec state.

Have lua_setup return the state it creates. Store the state created from
--lua-exec in the global options table. Use a temporary local for
--lua-exec-internal.
Don't use global o.cmdexec in lua_setup.

Pass the name of the file to read as a parameter.