* daemon,tests: support forgetting device serial via API
this is done by posting {"action":"forget"} to /v2/model/serial
a flag no-registration-until-reboot is also supported
* many: more consistent naming Delete => DeleteByName on keypair mgrs
we actually want to introduce a Delete by key id on some of them now
* asserts: implement KeypairManager.Delete
* devicestate: Unregister deletes the device key pair as well
* tests: test device key deletion in generic-unregister
* asserts: avoid skipping the GPGKeypairManager.Delete test
pair --yes to --batch in the test
