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: I5deeb4dcf6d3828cef3a879bf2aac7e8da6010b1
Reviewed-on: https://code.wireshark.org/review/4766
Reviewed-by: Guy Harris <guy@alum.mit.edu>