further dns_tsigkey API tweaks

further dns_tsigkey API tweaks

- remove the 'ring' parameter from dns_tsigkey_createfromkey(),
  and use dns_tsigkeyring_add() to add key objects to a keyring instead.
- add a magic number to dns_tsigkeyring_t
- change dns_tsigkeyring_dumpanddetach() to dns_tsigkeyring_dump();
  we now call dns_tsigkeyring_detach() separately.
- remove 'maxgenerated' from dns_tsigkeyring_t since it never changes.