Start using PAM headers; improve owned/borrowed distinction. - Uses bindgen to generate bindings (only if needed). - Gets the story together on owned vs. borrowed handles. - Reduces number of mutable borrows in handle operation (since `PamHandle` is neither `Send` nor `Sync`, we never have to worry about thread safety. - Improves a bunch of macros so we don't have our own special syntax for docs. - Implement question indirection for standard XSSO PAM implementations.