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

This avoids locale-dependent tests.

Change-Id: I709ff6b1a837fb0390b29f911d3b9ef1015a2d1e
Reviewed-on: https://code.wireshark.org/review/4758
Reviewed-by: Guy Harris <guy@alum.mit.edu>