On a server without a backup key, backup:target:add finds the backups at
the location and takes their key from its config with one of the old
server's passphrases. Backups then wait until a new recovery kit is
confirmed. The deletion delay and the repository id are now stored on the
location, sealed with the key, so the delay survives a restore without a
kit and --repair-config can write a lost config again from the kit.
The key page warns against creating a new key when restoring a server.
New test: tests/integration/recovery-scenarios.sh, and a no-kit mode in
full-restore-roundtrip.sh.
