Safer defaults for immutable znc config (#30155)

* Safer defaults for immutable znc config

I just lost all the options I configured in ZNC, because the mutable config was overwritten.
I accept any suggestions on the way to implement this, but overwriting a mutable config by default seems weird. If we want to do this, we should ensure that ZNC does not allow to edit the config via the webmin when cfg.mutable is false.

* Do not backup old config files.

There seems to be little need for backups if mutable becomes a voluntary opt-out.

* fixup