gopts: check/ plugins perform no validation during kdbGet()

[qwepoizt]

Forked from #4029.

This is "Class 3: validation errors reported by check/* plugins".

Steps to Reproduce the Problem

see #4029

Expected Result

see #4029

Actual Result

see #4029

System Information

see #4029

[qwepoizt]

Summary of this issue from #4029:

• The check/* plugins (e.g. range, validation) only perform their validation during elektraPLUGINNAMESet() and not during elektraPLUGINNAMEGet(). The ...Get() functions pass the unchanged KeySet through.
• Some check/* plugins (e.g. check/validation) are currently missing the postgetstorage placement. In order for their elektraPLUGINNAMEGet() to actually be called during kdbGet(), we'll have to add that placement.

[qwepoizt]

See #4053 for further discussion.

[qwepoizt]

Closing this as the discussion has moved to #4053 and a new issue for all plugins has been opened in #4059.