Don't omit CN verification in DarwinSSL when an IP address is used.

On OS X, curl does not output any sort of warning or error if an https request is made with an IP address rather than a hostname, no matter what the CN field in the SSL certificate is. This is because the SSLSetPeerDomainName() call is required even for basic hostname checking. It also performs SNI checking if available.

This fix prints a warning but allows the connection to go through, which matches curl's behaviour when compiled on Windows with SChannel. Most of the other vtls backends do verify the IP address against the CN field. If this is preferred, changing the behaviour here to be consistent would be trivial. Ideally, all of the vtls backends would be consistent — this would be fairly easy to fix, but I am unable to test some of the platform-specific ones. I'm unsure which approach to take, as having an IP address in the CN field does not appear to be proper.

(This appears to be the cause of the issue mentioned at the bottom of https://www.imperialviolet.org/2014/02/22/applebug.html which threw people off for a while.)