Apple Mac OS X Keychain support

Hello,

This is a resurrection of the original patch submitted by Bran Raderman <brian@irregularexpression.org> four years ago. I took the liberty of porting the patch to the current OpenVPN source style and layout, while at the same time adapting it to follow in the footsteps of the Microsoft CryptoAPI support as closely as possible.

This has been tested to work fine on Mac OS X 10.9.1 using a binary compiled using the XCode 4.x command line tools and GNU autotools installed using brew.

Please have a look,
Kind regards,
Radu - Eosif Mihailescu