DSGL proposal: drop criterion that init commit be signed

[castedo]

Currently Hidos will error if the initial commit of a signed succession is not signed. And the DSGL spec 1.1 has this as a criterion for an ungarbled succession.

I propose that it is OK for the initial commit to be unsigned, even for a signed succession. It is still a hard criterion that all non-initial commits be signed with an allowed key.

But this really isn't necessary. It really only maters that non-initial commits are signed with whatever keys are listed in the init commit.

I originally made Hidos error figuring that it is a good way to make sure a user has their signing key setup properly. But on second thought I don't this is worth the extra complexity and artificial constraint.