network: enable DHCP broadcast flag if required by interface

network: enable DHCP broadcast flag if required by interface

Some interfaces require that the DHCPOFFER message is sent via broadcast
if they can't receive unicast messages before they've been configured
with an IP address.

E.g., s390 ccwgroup network interfaces operating in layer3 mode face
this limitation. This can prevent the interfaces from receiving an
IP address via DHCP, if the have been configured for layer3.

To allow DHCP over such interfaces, we're introducing a new device
property ID_NET_DHCP_BROADCAST which can be set for those.
The networkd DHCP client will check whether this property is set
for an interface, and if so will set the broadcast flag, unless
the network configuration for the interface has an explicit
RequestBroadcast setting.

Besides that, we're adding a udev rule to set this device property
for ccwgroup devices operating in layer3 mode, which is the case
if the ID_NET_DRIVER property is qeth_l3.

Supercedes #18829