Check for printable ASCII in an ENBName.

I'm not sure why the check is only for alphabetical characters; if
that's correct, change this to use g_ascii_isalpha, and change
is_printable_ascii to is_ascii_alpha or something such as that.

Don't use ctype.h routines, as they are locale-dependent.

Change-Id: I61d0672350d35ad918e95d7e96ed5dd263102da9
Reviewed-on: https://code.wireshark.org/review/4805
Reviewed-by: Guy Harris <guy@alum.mit.edu>