Ref T13098. Add ^C handling and some small bits:
- Update arc weld.
- Test that arc weld filen<tab> completes filename (it does).
- Add a "workflow stack" -- I plan to make it easier for arc diff to call arc unit / arc lint as formal sub-workflows, etc., and make "workflow X delegates to workflow Y" a more formal thing.
- Workflows can do something when you press ^C.
- If they do, press ^C twice quickly to exit.
- Otherwise, we exit on the first ^C.
The major thing I'd like to do in the short-ish term is to make phage report status on interrupt, but some other workflows might make sense to have interrupt handlers (maybe long-running stuff like arc upload / arc download) and third parties may find creative uses for them.