[confcom] Add warning when stdio enabled

[DomAyre]

If confcom is invoked in a way which produces a policy which enables stdio on the container, we should warn the user that this is what they're doing to avoid mistakenly generating policies which are too permissive.

We have an option to --disable-stdio, since the default may change in the future, we should also introduce an option to explicitly enable stdio if that's what the user wants.

[yonzhan]

Thank you for opening this issue, we will look into it.

[DomAyre]

Done in

• [confcom] Add a warning and path for default change for stdio #9203