The various character predicates (isspace and the like) may not be

The various character predicates (isspace and the like) may not be
used with negative char values, so I've added casts to unsigned char.
Maybe what really should be done is change all those arrays and
pointers to type unsigned char [] or unsigned char *, respectively;
but using plain char with those predicates is just wrong, so something
had to be done.
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