This will allow us to try multiple network managers in sequence.

This causes us to look up the available interfaces on every try, rather
than just once.  Interface lookup shouldn't be expensive, and recognizing
new interfaces is probably useful anyway.

Check both NetworkManager and networkd on each retry iteration.  The
NetworkManager check works with both dhclient and NM's internal DHCP
client.  Avoids a 30-second boot delay on Azure and AzureStack as we time
out waiting for a networkd lease.  Doesn't help in the initrd on RHCOS 8,
since NetworkManager doesn't daemonize there.

Support zbus 2.3 because Fedora isn't shipping zbus 3 yet.  The two are
API-compatible for our purposes.

Tested on Azure on Flatcar and RHEL in the real root, and FCOS in the
initrd.  Not tested on CloudStack, except to verify that NetworkManager
exposes a dhcp_server_identifier DHCP option with the expected format.

Fixes #146.