Minor refactor of Ctrl-C handling

[deivid-rodriguez]

This is the same set of changes as #6452, but actually keeping the "2-phase" exit. Basically a minor refactor that moves the full Ctrl-C handling implementation to the runner, so it's easier to follow and understand.

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

Thanks!