"chocpkg update" command

[fragglet]

Given that we now have "latest" variants to build from version control head, it would be good if there was some way to just automatically update all packages that have been previously built and installed. This will require keeping some kind of "database" of what packages are currently installed, and going through them all to trigger rebuilds.

But really there are several different use cases here that this should probably cover:

1. User previously built the "latest" variant of a package and wants to keep it up to date with the latest version (through git pull, etc.)
2. User previously installed a package - either explicitly or as a dependency, then updates the chocpkg repository; the version installed by chocpkg has changed and there is a newer version that should be installed.

Use case 2 covers not just the "built from git" case but also the stable "download a release" one (ie. fetch_download). This will be important in the future if we want to increase the minimum version of dependent libraries for Chocolate Doom or other packages.