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: I51d9716fe3eb02a6e98208334285c07597a6be79
Reviewed-on: https://code.wireshark.org/review/4761
Reviewed-by: Guy Harris <guy@alum.mit.edu>