* Join the IPv4 all-hosts multicast group 224.0.0.1 once only;
that is, when an IPv4 address is first configured on an interface.
* Do not join it for subsequent IPv4 addresses as this violates IGMP.
* Be sure to leave the group when all IPv4 addresses have been removed
from the interface.
* Add two DIAGNOSTIC printfs related to the issue.
Further care and attention is needed in this area; it is suggested that
netinet's attachment to the ifnet structure be compartmentalized and
non-implicit.
Bug found by: andre
MFC after: 1 month