Fix CA certificate bundle handling in darwinssl.

If the --cacert option is used with a CA certificate bundle that
contains multiple CA certificates, iterate through it, adding each
certificate as a trusted root CA.
Check CA certificate in curl_darwinssl.c.

SecCertificateCreateWithData() returns a non-NULL SecCertificateRef even
if the buffer holds an invalid or corrupt certificate. Call
SecCertificateCopyPublicKey() to make sure cacert is a valid
certificate.
Merge pull request #115 from ldx/darwinsslfixpr

darwinssl: now accepts cacert bundles in PEM format in addition to single certs