darwinssl: add TLS crypto authentication

Users using the Secure Transport (darwinssl) back-end can now use a
certificate and private key to authenticate with a site using TLS. Because
Apple's security system is based around the keychain and does not have any
non-public function to create a SecIdentityRef data structure from data
loaded outside of the Keychain, the certificate and private key have to be
loaded into the Keychain first (using the certtool command line tool or
the Security framework's C API) before we can find it and use it.