dhcpv4: log requests and replies to syslog

This change let odhcpd print dhcpv4 events
to syslog.
dhcpv4: implement router configuration option

RFC 2132 defines:

3.5. Router Option

   The router option specifies a list of IP addresses for routers on the
   client's subnet.  Routers SHOULD be listed in order of preference.

   The code for the router option is 3.  The minimum length for the
   router option is 4 octets, and the length MUST always be a multiple
   of 4.

    Code   Len         Address 1               Address 2
   +-----+-----+-----+-----+-----+-----+-----+-----+--
   |  3  |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |  ...
   +-----+-----+-----+-----+-----+-----+-----+-----+--
Merge pull request #20 from mehlis/implement-dhcp4-router-config

dhcpv4: implement router configuration option
dhcpv4: offer a valid configuration with DHCP NAK

On network change the client requests a non valid
configuration. In this case we have to reply with
a DHCP NAK. But this NAK can include a valid
configuration like an ACK does.

With this change iDevices and Androids can finish
DHCP even with wrong configuration in first
REQUEST with two packets and without any additional
timeouts and round trips.
Merge pull request #21 from mehlis/fix-nak-by-doing-valid-reply

dhcpv4: offer a valid configuration with DHCP NAK
Merge pull request #22 from mehlis/log-dhcp4-events

dhcpv4: log requests and replies to syslog
PD: don't consider deprecated assigned prefixes when calculating avilable PD size