Being able to back up the hsm_secret is critical, but you cannot do
this through a UI, because of course we do not allow such access.
People have lost funds because they didn't back up.
This allows access to the hsm_secret if you use a password set in the
config file. (If it's not set, the command does not work). This is a
compromise, of course.
Changelog-Added: `exposesecret` command for encouraging hsm_secret backups.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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