new usecase: allow swapping of login and alt_login fields

The way the checks are currently done, one cannot swap the login and alt_login field in one go.
This is useful, since the normal login field is the `canonical' author name.

The appended patch makes more precise requests to allow for alt_login to be the login for the same uid and vice-versa.

It relies on alt_login vs. login comparison to make certain we can't set both to the same value.

i don't think I missed anything, but still it needs review...