Slightly hacky but hopefully functional fix for KDMs after the changes

Slightly hacky but hopefully functional fix for KDMs after the changes
to signer.  The KDM stuff uses add_signature_value() but not sign()
since it has to allow pass-through of a KDM (so it handles <Signature> etc.
itself).  This means we have to make the indentation-adding optional.

It might have been nicer to make indent() not add indentation if it's
not already there.