scripts/set-version: Re-process vulns.git for the given branch

set-version needs should properly recheck the status of each issue on the update branch, rather than applying the simple heuristic it does now. In particular, it doesn't currently handle issues that are introduced on a stable branch.

Move much of import-korg-vulns into a korg_vulns module, and change set-version to call that.

Add command-line arguments to set-version to specify the Linux repository to look at.