Added headless guided package installation

Added support for headless guided installation of `pkg` and `mpkg` files; see https://github.com/sparkle-project/Sparkle/issues/348. This allows the installation of Installer packages without a user interface. This method also allows one application to update a suite of tools through an Installer package.

Ported existing code to use ARC and inlined the documentation. The original code has been well tested and used in commercial products for many years. That code supported 10.4 and later. I have minimised the changes to avoid introducing bugs or problems. I expect further changes can be made given the shift to supporting 10.7+.

Ideally, the use of `AuthorizationExecuteWithPrivileges` should be replaced with an XPC or an external authtool like process.