child-rekey: Use previously negotiated DH group when rekeying CHILD_SAs

child-rekey: Use previously negotiated DH group when rekeying CHILD_SAs

For the CHILD_SA created with the IKE_SA the group won't be set in the
proposal, so we will use the first one configure just as if the SA was
created new with a CREATE_CHILD_SA exchange.  I guess we could
theoretically try to use the DH group negotiated for IKE but then this
would get a lot more complicated as we'd have to check if that group is
actually contained in any of the CHILD_SA's configured proposals.