avahi-autoipd: only consider ARP Probe packets

[chhitz]

during the probe phase avahi-autoipd must only select a new IP when
it receives an ARP Probe packet:

According to RFC3927, Section 2.2.1. Probe details:

An ARP Request constructed this way with an all-zero 'sender IP
address' is referred to as an "ARP Probe".

and

In addition, if during this period [checking if an address is already
in use] the host receives any ARP Probe where the packet's 'target IP
address' is the address being probed for, and the packet's 'sender
hardware address' is not the hardware address of the interface the
host is attempting to configure, then the host MUST similarly treat
this as an address conflict and select a new address as above.

When receiving an ARP packet during probe avahi-autoipd did not check
if the 'sender IP address' is all-zero and therefore if the packet is an
ARP Probe.

refs #6