darwinssl: add TLS 1.1 and 1.2 support, replace deprecated functions

In Mountain Lion, Apple added TLS 1.1 and 1.2, and deprecated a number
of SecureTransport functions, some of which we were using. We now check
to see if the replacement functions are present, and if so, we use them
instead.  The old functions are still present for users of older
cats. Also fixed a build warning that started to appear under Mountain
Lion