Pass 'unsigned char' type to isdigit()/isspace()/isprint() functions

The isdigit(), isprint(), etc. functions take an int, whose value is
required to be in the range of an _unsigned_ char, or EOF.  This, horribly,
means that systems which have a signed char by default need casts to pass
a char variable safely to these functions.

We can't do this more nicely by making the variables themselves 'unsigned
char *' because then we'll get warnings passing them to the strchr() etc.
functions.

At least the cygwin version of these functions, are designed to generate
warnings if this isn't done, as explained by this comment from ctype.h:
   These macros are intentionally written in a manner that will trigger
   a gcc -Wall warning if the user mistakenly passes a 'char' instead
   of an int containing an 'unsigned char'.

Signed-off-by: Serge Lamikhov-Center <Serge.Lamikhov@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>