Add support for --cacert in DarwinSSL.

Security Framework on OS X makes it possible to supply extra anchor (CA)
certificates via the Certificate, Key, and Trust Services API. This
commit makes the '--cacert' option work using this API.

More information:

https://developer.apple.com/library/mac/documentation/security/Reference/certifkeytrustservices/Reference/reference.html

The HTTPS tests now pass on OS X except 314, which requires the '--crl'
option to work.