Difficulty_level_type is used in arithmetic expressions. If
Difficulty_level_type is unsigned, then those expressions use unsigned
terms, but some of the expressions were designed to use signed terms and
produce incorrect results when used with unsigned terms. There is no
strong reason to make Difficulty_level_type unsigned, so switch it to
signed instead of trying to fix every site that uses it.
Reported-by: AlumiuN <https://github.com/dxx-rebirth/dxx-rebirth/issues/471>
Fixes: 1eaaff3016 ("Move Difficulty_level to GameUniqueState")
075b14373a