Explicitly use `signed char` or `unsigned char`, rather than
```
signed char k;
char k;
signed char k;
char k;
```
This should have resulted in the same thing; however, note that
`REFCC` was never defined, and `common.h` was not included, so
the old code in fact tested `char` then `unsigned char`,
which are the same.
The only difference is that a switch using plain `char` is not
tested, but since this is the same as either `signed char` or
`unsigned char`, the lack of test coverage seems relatively safe.
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