Don't use ctype.h routines.

That avoids locale dependency and handles possibly-signed chars (which
we weren't doing before).

Change-Id: I70f3d993c9a8fbf870901f12b430d733968c3fa8
Reviewed-on: https://code.wireshark.org/review/4781
Reviewed-by: Guy Harris <guy@alum.mit.edu>