The Lua C API does not modify errno, so we don't have to save it.

A common idiom I was using was:

	if (someerror)
	{
	  int err = errno;
	  lua_pushnil(L);
	  lua_pushinteger(L,err);
	  return 2;
	}

But upon investigation, the Lua C API does not modify the contents of errno,
so we can save a few bytes here and there by changing instances of the above
to

	if (someerror)
	{
	  lua_pushnil(L);
	  lua_pushinteger(L,errno);
	  return 2;
	}

Any remaining idioms like that are due to calls to other than the Lua API
between saving errno and using its result.
Remove now dead variable.