Fix errors after deciding on the Register API.
Fix errors after deciding on the Register API.

The ParamsForNet function was removed, so likewise this change removes
ErrUnknownNet error that it used to return.

As network registration is now necessary for correct handling of
alternate network encoding magics, and therefore the ErrDuplicateNet
error returned by Register is here to stay, kill the comment about the
error being removed later.

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