warnings from use of ctype.h methods

[DJ Delorie]

In the case where you have a string, and you're explicitly checking for
NUL elsewhere, the right thing to do is to map the data you have into
the domain the function wants.  In this case, a cast to (unsigned char)
gives your data the correct interpretation for the is*() functions.

If you cast a plain char to an int, you get values in the set
[-128..127], which is not the domain of values the is*() functions
expect.  You should provide the is*() functions values in the set
[0..255, EOF].