provide tooling to create config file from shell

[ilka-schulz]

I would like to see config files created by shell scripts. For this, two different features could be implemented:

1. provide a command that checks a given config file whether it is valid (like sshd -t)
2. provide a command to edit a given config file and refuse invalid edits (like postconf -e)

[pqcfox]

Would it make sense to roll these into a family of rosenpass config subcommands like those suggested in #241? If so, I'll try that out after I get a draft PR up for #241 itself.