IPv4: Remember if we advertised an address

IPv4: Remember if we advertised an address

Similar to IPv6.

However, this code will just live in this branch and go no futher
because it's not tennable. Modern DHCP servers won't let you
assign the same IP to different hardware addresses.
Some kernels announce ARP after a link flap giving a large window
where requests could goto a valid address but the wrong one.

What IP address sharing set out to achieve was a poor mans link
aggregation for OSs that lacked another way of doing it ....
Well, with NetBSD finally getting lagg(4), all supported OS's
now have this so there is no more need for it in dhcpcd.