AppleTLS: Implement native TLS support on OSX

As mentioned in issue #56, by @antbryan and others, OSX provides platform SSL/TLS support.
I took the opportunity to implement just that in aria2. As AppleTLS uses the KeyChain, it will also automatically use the root CAs stored there, so no more worries about ca-bundle, like the stuff @zhuangya encountered.
Besides that, I also implemented message digests via OSX platform support.

This stuff is still somewhat experimental. It should be default secure, but I'm not an expert :p
Maybe mark it experimental for now and require explicitly configuring with --with-appletls?

Also, the configure stuff still is somewhat crappy. As I didn't implement the RC4/DH stuff, it still needs gcrypt/nettle/gmp/openssl. But openssl is disabled when appletls is enabled, so one less option. Not sure about that...

IIRC this stuff should build and work in OSX 10.4 and later.

Feel free to comment, but please do not merge before I had a chance to test more compiler configs and SSL scenarios