style: argparse inheritance, hide some options from --help

[pajod]

We have way too much options (#3266)

Idea: We could hide deprecated ones from --help, and possibly move both unstable and deprecated ones to their own, bottom-most section in the documentation.

Attached patch hacks (style improvements welcome) the config.py-class-to-argparse code so we can subclass while keeping metaclass intact. Then it adds a Deprecated class that hides certain settings from --help. I tried to avoid additional functional changes.

If this PR moves forward with minor changes, then the following changes are the next steps:

1. change some options default value to None and have not specifying them mean "feature not used"
   • --user/--group unset should mean do not do that - see clearly point out the surprising behavior of --reload, --user, --group, --config (+ unlock a few previously impossible combinations) #3360
2. permit and document some options with existing default to be overridden to some None-like value
   • --config="" should mean do not read config (as in: --config=/dev/null) - see clearly point out the surprising behavior of --reload, --user, --group, --config (+ unlock a few previously impossible combinations) #3360
3. options that do not take effect based on environment / other options should be warned about
   • behavior of --bind fd://3 when LISTEN_FDS=1 set can be erratic & surprising
4. boolean options should use argparse.BooleanOptionalAction (Python 3.9+)
   • paves the way to switch settings default-off to default-on without forcing admins to update their config

• Suggested merge order: after any feature/bugfix PRs. merge conflicts more easily solved within this PR.
• fixes: Does the tmp_upload_dir setting actually do anything? #3349

[benoitc]

Closing this PR as it's primarily a style change that has been open for over a year. Thank you for the contribution.