libsystemd-dhcp: Handle T1 Renewing timeout

libsystemd-dhcp: Handle T1 Renewing timeout

Expiration of T1 timeout takes the client to the Rebinding state,
where it attempts to renew its lease. Start by opening a DCHP unicast
socket as there now is a proper IP address set.

Compute the resend timer as half of the remaining time down to a
minimum of 60 seconds (RFC2131). Modify DHCP Request sending to send
only UDP DHCP data when unicasting. Also modify DHCP Ack/Nak
receiving such that the client_receive_ack() takes care of using
either the full IP, UDP, DHCP packet or only the DHCP payload
depending whether the client is in Requesting or Renewing state.
Finally always report DHCP_EVENT_IP_ACQUIRE from Requesting state
and only DHCP_EVENT_IP_CHANGE if the IP address was modified when
the lease was renewed.