userdb: when loading a user record from JSON, mark 'secret' section in JSON variant...

userdb: when loading a user record from JSON, mark 'secret' section in JSON variant as sensitive as side effect

When we load a user record we retain a reference to the original JSON.
Thus the loaded objects might live at least as long as our user record
object, hence we better make sure we set the 'sensitive' flag for the
'secret' section if it's not marked like that yet.

This is paranoia only: we already should be setting this flag properly
earlier, when acquiring the json variant in the first place. But it's
better to be safe than sorry.