networkd: link - use IFF_DORMANT instead of IFF_RUNNING

Should be no functional change, but avoids spurious log
messages when in operstate UNKNOWN and !IFF_LOWER_UP.
networkd: link - log changes in some common link flags
sd-dhcp-client: log ifindex and mac address when starting
networkd: link - set mac addresses when starting clients

MAC addreses are also set when NEWLINK messages are receieved, but only if the clients have
already been initialized.
networkd: link - add explicit unmanaged state