We cannot use the codex32 or raw hex for recovery of 25.12 nodes,
since they will then use the incorrect derivation for all paths, and
be unable to spend (or even find!) their funds.
So implement `getsecret` to replace `getcodexsecret`.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Changed: `lightning-hsmtool`: `getsecret` replaces `getcodexsecret` for modern nodes (gives mnemonic).
Changelog-Deprecated: `lightning-hsmtool`: `getcodexsecret`. Use `getsecret`.