userwork: properly handle ENOLINK error from lower-level userdb code

userwork: properly handle ENOLINK error from lower-level userdb code

The lower-level userdb code will return ENOLINK if varlink lookups are
disabled explicitly and we couldn#t find an answer any other way. Let's
not propagate this to clients, since they don't have control over this
feature anyway: we decide internally when to disable varlink lookups
(e.g. if DropIn lookups are requested we disable them) and to the client
side that should not be visible: if we can't find a record with the
flags we pick then we should report then we can't find any, and that's
it.

Fixes: #21223