Document CVMFS authorization protocol.

[bbockelm]

The CVMFS authorization protocol probably really only "exists" in the form of source code.

We should reverse-engineer / read the sources and create a MarkDown-formatted doc of the various RPCs and the responses.

[bbockelm]

Here's the reference implementation using X509: https://github.com/cvmfs/cvmfs-x509-helper

[bbockelm]

A few salient points:

• Data is always read from stdin and written to stdout. Everything is single-threaded and blocking (meaning we don't have to ).
• Immediately after exec'ing the helper process, a handshake message is sent: https://github.com/cvmfs/cvmfs-x509-helper/blob/master/src/x509_helper.cc#L92
• The client should respond with a handshake: https://github.com/cvmfs/cvmfs-x509-helper/blob/master/src/x509_helper.cc#L176
• After handshaking, the process should enter an event loop of receiving authorization requests and responding: https://github.com/cvmfs/cvmfs-x509-helper/blob/master/src/x509_helper.cc#L181
• Here's an example parser for the auth request: https://github.com/cvmfs/cvmfs-x509-helper/blob/master/src/x509_helper.cc#L122
• Here are the examples of how to respond: https://github.com/cvmfs/cvmfs-x509-helper/blob/master/src/x509_helper.cc#L199