caddytls: fix preferred chains options by appending values instead of replacing

[okrc]

{
  preferred_chains {
    root_common_name "Example root name"
    root_common_name "Example root name 2"
  }
}

I mainly noticed that when I write multiple lines for root_common_name, the last line always overrides the previous ones, and there is no warning or indication that this is not allowed. I attempted to modify it so that multiple lines can be used. Thank you for all your work on this project.

Assistance Disclosure

No AI was used.

[CLAassistant]

All committers have signed the CLA.

[francislavoie]

That's fine I guess, typically we throw an error if trying to fill an already-filled value, sometimes we allow repeated values. In this case I think you could've put all the values in sequence on one line. But I can see the argument for allowing it on multiple lines too.