Only generate a new xid on state change.

Only generate a new xid on state change.

When we first request a lease (INIT or REBOOTING state) we run with
very short timeouts. If the dhcp server is slow to respond we already
have a new xid and ignore the server's response. This goes on until we
increase the timeout high enough. If we just stick to an xid this will
not happen and we accept "late" responses.

RFC 2131 has:
Selecting a new 'xid' for each retransmission is an implementation
decision.  A client may choose to reuse the same 'xid' or select a new
'xid' for each retransmitted message.

Problem seen by phessler on german train wifi.
OK phessler