darwinssl: enable BEAST workaround on iOS 7 & later

iOS 7 finally added the option to enable 1/n-1 when using TLS 1.0
and a CBC cipher, so we now always turn that on unless the user
manually turns it off using CURLSSLOPT_ALLOW_BEAST.

It appears Apple also added some new PSK ciphers, but no interface to
use them yet, so we at least support printing them if we find them.