Don't use ctype.h routines.

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

Change-Id: Ieceb93029252f646397b6488f2df8a57c6d2a23d
Reviewed-on: https://code.wireshark.org/review/4794
Reviewed-by: Guy Harris <guy@alum.mit.edu>