Previously, a static lease could be configured only with MACAddress=, which matches a client by its
hardware address. Hosts that share a hardware address, e.g. ipvlan interfaces in L2 mode, but send
different client identifiers could not get different static addresses.
This introduces ClientIdentifier=, IAID=, DUIDType=, and DUIDRawData= settings, with the same
semantics as the ones in the [DHCPv4] section. With ClientIdentifier=mac, the default, the client
identifier is built from MACAddress= as before. With ClientIdentifier=duid, MACAddress= is ignored,
and the client identifier is built from IAID= and the DUID specified by DUIDType= and DUIDRawData=,
all of which must be specified.