The `--check-config` tool should not report (some) errors for private items

[almostearthling]

Private items in some cases might exploit features of whenever that When does not allow: in such cases the --check-config tool should not report values that When considers incorrect as errors. Neverthertheless it should report errors introduced by a user that incorrectly modified any private item.

[almostearthling]

Introduction of specific checks on standard items would be confusing an already complex codebase for item management: for the specific case that raised the issue (the "need" to use an initial condition, that is, an interval based condition that is only checked once and at an interval of 0 seconds), two options can be checked:

1. use an interval of 1 second, which is initial anyway most of the times, since the default tick period is 5 seconds,
2. create an InitialCondition item, not available for the user, and used to start initial tasks that are useful to prepare the environment for private When items.

While the first option is easy to implement, maybe it is too simplistic. And "most of the times" is hardly acceptable.

However, no exceptional checks will be performed in standard items.