At the moment only lightingd needs it, and this avoids missing any
places where we do bip32 derivation.
This uses a hsm capability to mean we're backwards compatible with older
hsmds.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: Protocol: we now always double-check bitcoin addresses are correct (no memory errors!) before issuing them.