Fix luacheck warnings

[daurnimator]

These all seem to be true issues:

$ luacheck --codes mojibake.lua 
Checking bench.lua                                13 warnings

    bench.lua:8:7: (W211) unused variable bnot
    bench.lua:8:13: (W211) unused variable band
    bench.lua:8:24: (W211) unused variable bxor
    bench.lua:8:30: (W211) unused variable lshift
    bench.lua:8:38: (W211) unused variable rshift
    bench.lua:11:10: (W111) setting non-standard global variable readfile
    bench.lua:21:10: (W111) setting non-standard global variable writefile
    bench.lua:75:8: (W211) unused variable EXIST_SUCCESS
    bench.lua:107:8: (W311) value assigned to variable found_option is unused
    bench.lua:110:18: (W113) accessing undefined variable readfile
    bench.lua:118:10: (W213) unused loop variable k
    bench.lua:127:15: (W113) accessing undefined variable writefile
    bench.lua:136:10: (W113) accessing undefined variable EXIT_SUCCESS

Checking mojibake.lua                             10 warnings

    mojibake.lua:47:24: (W211) unused variable bxor
    mojibake.lua:16336:3: (W111) setting non-standard global variable index_fn
    mojibake.lua:16337:8: (W111) setting non-standard global variable index_fn
    mojibake.lua:16340:14: (W113) accessing undefined variable index_fn
    mojibake.lua:16347:14: (W113) accessing undefined variable index_fn
    mojibake.lua:16355:12: (W113) accessing undefined variable index_fn
    mojibake.lua:16364:12: (W113) accessing undefined variable index_fn
    mojibake.lua:16375:12: (W113) accessing undefined variable index_fn
    mojibake.lua:16471:10: (W113) accessing undefined variable append_characters
    mojibake.lua:16478:10: (W113) accessing undefined variable utf

Checking normtest.lua                             26 warnings

    normtest.lua:8:10: (W111) setting non-standard global variable CHECK_NORM
    normtest.lua:18:10: (W111) setting non-standard global variable main
    normtest.lua:19:12: (W311) value assigned to variable buf is unused
    normtest.lua:20:12: (W211) unused variable bufsize
    normtest.lua:48:11: (W311) value assigned to variable offset is unused
    normtest.lua:50:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:51:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:52:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:53:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:54:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:56:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:57:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:58:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:59:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:60:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:62:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:63:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:64:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:65:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:66:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:68:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:69:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:70:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:71:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:72:11: (W113) accessing undefined variable CHECK_NORM
    normtest.lua:80:1: (W113) accessing undefined variable main

Checking unicode_test.lua                         OK

Total: 49 warnings / 0 errors in 4 files