Eliminate use of ctype.h routines.

That way, we don't do locale-sensitive case-insensitivity (yes, the
locale can affect case-mapping - in a Turkish locale, "I" isn't the
upper-case version of "i", for example).

Change-Id: I5f7663e85160558ff3769617f924e45049c9c384
Reviewed-on: https://code.wireshark.org/review/4843
Reviewed-by: Guy Harris <guy@alum.mit.edu>