libsystemd-network: Prepend hardware type byte to client identifier

Even though client identifiers SHOULD be treated as opaque objects by
DHCP servers, follow the recommendation of a hardware type field with
value 0x01 (ethernet) followed by the hardware address as described in
RFC 2132.
libsystemd-network: Don't unnecessarily send too long packets

Since the length used by options is known, send packets with no
extra padding.
libsystemd-dhcp: Update client test case for client id and end option

Check that the client identifier is formatted as suggested in the
RFC and that the messages sent ends with an end option.