I feel like this should be simpler:

  MISH_CMD_NAMES(set, "set");
  MISH_CMD_HELP(set,
    "set 'cnt' variable",
    "test command for libmish!");
  MISH_CMD_REGISTER(set, _test_set_cnt);

Should be

  MISH_CMD(set,
           _test_set_cnt,
           "set 'cnt' variable",
           "test command for libmish!")

Which can expand as needed.