Use explicit AdminUsers check in SetPrivateKey, not _Accessible

8ce033b3's addition of it to _Accessible was to leverage the code in
->CurrentUserCanModify, which is called by SetPrivateKey.  The presence
of PrivateKey in _Accessible leads to problems in other code (such as
rt-dump-metadata) which assumes that _Accessible only contains true
columns.

Switch to an explicit ACL check in SetPrivateKey rather than inserting
into _Accessible and relying on CurrentUserCanModify.

Fixes I#22465.
Merge branch '4.0/dump-metadata-privatekey' into 4.0-trunk