dhcp: support endianness independent dhcp_identifier_set_iaid()

dhcp: support endianness independent dhcp_identifier_set_iaid()

The previous code did htole64() followed by unaligned_write_be32() (the
XOR and shift in between is endianness agnostic). That means, on every
architeture there is always exactly one byte swap and the iaid is
dependent on endianness.

Since dhcp_identifier_set_iaid() is part of the DUID generation
algorithm, this cannot be fixed without changing the client-id.
In particular, as the client-id already depends on the machine-id (and
is thus inherrently host-specific), it is better to stick to the current
behavior.

However, add a parameter to switch between old and new behaviour.
Since the new behavior is unused, the only real purpose of this
change is to self-document the oddity of the function.

Fixes: 933f9caeeb2b3c1b951d330e04beb04226e5a890