Do not create ARP entries for RTF_BROADCAST routes.

Do not create ARP entries for RTF_BROADCAST routes.

This has been done because historically routes to broadcast addresses
were cloned like any ARP entry.  But for obvious reasons, no matching
Ethernet address could ever be resolved.  That's why we played tricks
with the expire timer.

Now that a RTF_BROADCAST route is created per configured IPv4 address,
we need to differenciate duplicated one.  And by not creating an ARP
entry we are allowed to write the IP address in the rt_gateway field,
which prevents MPATH conflicts.

This change is part of a fix to unbreak aliases since the kernel support
multiple connected routes for a subnet.

Found the hardway by djm@, ok claudio@