This allows tools to validate that it is accessing the correct hsm_secret for this node!
This is extremely important for backups: if they are using VLS, they need to back *that*
up instead, for example.
Changelog-Added: `hsmtool`: `getnodeid` command derives the node id from the hsm_secret, to verify it's the correct secret.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>