dhcp4: fix DHCP on InfiniBand interfaces

dhcp4: fix DHCP on InfiniBand interfaces

With these patches applied, networkd is successfully able to get an
address from a DHCP server on an IPoIB interface.

1)
Makes networkd pass the actual interface type to the dhcp client,
instead of hardcoding it to Ethernet.

2)
Fixes some issues in handling the larger (20 Byte) IB MAC addresses in
the dhcp code.

3)
Add a new field to networkds Link struct, which holds the interface
broadcast address.

3.1)
Modify the DHCP code to also expect the broadcast address as parameter.
On an Ethernet-Interface the Broadcast address never changes and is always
all 6 bytes set to 0xFF.
On an IB one however it is not neccesarily always the same, thus
fetching the actual address from the interface is neccesary.

4)
Only the last 8 bytes of an IB MAC are stable, so when using an IB MAC to
generate a client ID, only pass those 8 bytes.