Restore without the recovery kit: the key from the location, with a passphrase

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.

docs/HOWTO.md

@@ -157,6 +157,10 @@ occ backup:list offsite                              # the snapshots must be lis
 occ backup:restore:full offsite <snapshot>
 ```
 
+Without the kit, leave out `backup:key:import`: `backup:target:add` finds the backups and takes
+the key from the location with a passphrase (`--passphrase-file=/path/passphrase`, or it asks).
+Backups then wait until a new kit is downloaded and confirmed.
+
 The command refuses on an installation that already has more than one user, and asks before it
 starts; `--force` skips both (also needed to rerun it after it stopped halfway).