Better validation of config file

[mohe2015]

Subject of the issue

I think it would be great if

1. There would be a command to validate the config file (e.g. before reloading or restarting). Preferably this could be run without connecting to any ports etc. so you could run this in the build system (for NixOS)
2. Options that are not used would be detected so a misspelled security-important option would be detected
3. Also better validation of other options. E.g. it seems like an invalid provisioner type is ignored.

Your environment

• OS - NixOS
• Version - 2021-02-14 (branch systemd)

Steps to reproduce

n/a

Expected behaviour

Better validation of configuration file

Actual behaviour

Nonexisting options are ignored, some other options are also not validated correctly.

I didn't look in the code at all so I don't know how easy or hard this would be.

Crossref: NixOS/nixpkgs#112322