Add pathfinder support for certificate validation

Add pathfinder support for certificate validation

  From http://code.google.com/p/pathfinder-pki/

       PathFinder is designed to provide a mechanism for any program to
       perform RFC3280-compliant path validation of X509 certificates,
       even when some of the intermediate certificates are not present
       on the local machine. By design, Pathfinder automatically
       downloads any such certificates from the Internet as needed using
       the AIA and CRL distribution point extensions of the certificates
       it is processing. It has the ability to do revocation status
       checking either using CRL or OCSP, or both. And, given the recent
       vulnerabilities that have rendered the MD5 algorithm highly
       suspect, it allows the administrator to choose to not validate
       certificates using that algorithm anywhere in the trust path.

git-svn-id: http://opkg.googlecode.com/svn/trunk@261 e8e0d7a0-c8d9-11dd-a880-a1081c7ac358