Add self-check command

[matteodelabre]

We should add a toltecctl subcommand that diagnoses and tries to auto-fix common install issues such as:

• Missing wget-ssl package.
• Incorrect PATH value.
• Other things?

@Eeems proposed calling this subcommand toltecctl doctor. This command could be run automatically after toltecctl reenable.

[Eeems]

Maybe detect that rm2fb isn't running? And disable it and re-enable xochitl if it's failing to start?

Maybe also warn that no ssh key is configured?