Display StablePrivacy as `stable_privacy`

[Toorero]

My pull-request #149 has the wrong Display implementation present for In6AddrGenMode::StablePrivacy. @cathay4t stated its wish to in the pull-request submission process to be changed to stable_privacy (which I did) but somewhere along other changes made to the pull-request there was a regression.

[Toorero]

I could append the change to #172 or submit a new pull-request.