Don't include ctype.h and replace isascii() with _ascii_isprint().
Remove deprecated APIs while at it.

Change-Id: I002ed4a696782caaeeb70a3e4ced4ae81f3d5372
Reviewed-on: https://code.wireshark.org/review/4983
Reviewed-by: Anders Broman <a.broman58@gmail.com>