Don't use <ctype.h> macros, and eliminate an include of <ctype.h>.

This avoids locale-dependent tests, and fixes cases where we passed
signed char values to those macros (which is not safe with char being
signed, as it is on most, but not all, platforms).

Change-Id: Iea962cd7e4dfaf33c3c873ba43814cc6ed298d94
Reviewed-on: https://code.wireshark.org/review/4764
Reviewed-by: Guy Harris <guy@alum.mit.edu>