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).

(cherry picked from commit 63ab36be9266f79d92ac37e632299c8f0e2244f1)

Change-Id: I67a838946fc9637cfc826a830f6a64b3f8fa64b9
Reviewed-on: https://code.wireshark.org/review/4765
Reviewed-by: Guy Harris <guy@alum.mit.edu>