darwinssl: disable insecure ciphers by default

I noticed that aria2's SecureTransport code disables insecure ciphers such
as NULL, anonymous, IDEA, and weak-key ciphers used by SSLv3 and later.
That's a good idea, and now we do the same thing in order to prevent curl
from accessing a "secure" site that only negotiates insecure ciphersuites.