darwinssl: SSLv2 connections are aborted if unsupported by the OS

I just noticed that OS X no longer supports SSLv2. Other TLS engines return
an error if the requested protocol isn't supported by the underlying
engine, so we do that now for SSLv2 if the framework returns an error
when trying to turn on SSLv2 support. (Note: As always, SSLv2 support is
only enabled in curl when starting the app with the -2 argument; it's off
by default. SSLv2 is really old and insecure.)