Make ckan compat add take multiple versions, add clear and set

[HebaruSan]

Motivations

• Currently ckan compat add accepts only one version string argument, but it's common to need to add multiple versions (for example, we do exactly this in the metadata tester, and @JonnyOThan needs it for KSPBuildTools).
• The list, add, and forget operations of ckan compat do not make it easy to "get your compatible versions into a consistent reproducible state"

Changes

• Now ckan compat add accepts multiple version strings
• Now ckan compat forget accepts multiple version strings
• Now ckan compat clear is created to remove all compatible versions
• Now ckan compat set is created to overwrite the compatible versions with the given versions
• Now the commands that change the compatible versions display the compatible version list after the change is applied
• Now when you run a command with missing arguments, the resulting usage output is the same as the usage output in that command's --help page, rather than a separate ad hoc string

Fixes #4150.