Write better PAM documentation for the Linux PAM subsystem

[Sam Caldwell]

• The PAM subsystem is poorly documented.
• We can do better and help the community at large by explaining how PAM works
• Explain how PAM is configured.
• Identify areas where we can build tools to automate configuration.

[Sam Caldwell]

I did a bit of this on the wiki the other day. Much more is needed.