No need for our own wrappers around tolower() and toupper().

We can just use g_ascii_tolower() and g_ascii_toupper();

Change-Id: I8a88a096d16ce8c60dd9151e5bdddf6747702145
Reviewed-on: https://code.wireshark.org/review/4754
Reviewed-by: Guy Harris <guy@alum.mit.edu>