- if we leave the client-id of sd_dhcp_client unset, it will
  anyway generate a node-specific client-id (and may fail if
  "/etc/machine-id" is invalid).
  Anticipate that, and don't let the client-id unset. In case
  we have no client-id from configuration or lease, just generate
  the id ourself (using the same algorithm). The advantage is,
  that we know it upfront and can store the client-id in the
  NMDhcpClient instance. We no longer need to peel it out from
  the lease later.

- to generate the IPv4 client-id, we need a valid MAC address. Also,
  sd_dhcp_client needs a MAC address for dhcp_network_bind_raw_socket()
  as well. Just require that a MAC address is always needed. Likewise,
  we need a valid ifindex and ifname set.

- likewise for IPv6 and IPv4, cleanup detecting the arptype and
  checking MAC address length. sd_dhcp_client_set_mac() is overly
  strict at asserting input arguments, so we must validate them anyway.

- also, now that we always initialize the client-id when starting
  the DHCP client, there is no need to retroactively extract it
  again when we receive the first lease.