gracefully handle SIGINT, SIGTERM and SIGHUP

[oliver-sanders]

• Closes #4914
• Closes #6438
• If any of these signals are received, set the workflow to stop NOW.
• If the workflow is already stopping NOW, escalate that to NOW NOW (e.g. to handle stuck subprocpool processes)
• SIGINT is no longer handled as an exception (panic shutdown).

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• [x] Tests are included (or explain why tests are not needed).
• [x] Changelog entry included if this is a change that can affect users
• [x] Cylc-Doc pull request opened - https://github.com/cylc/cylc-doc/pull/768
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.