Switch to ifc_add_address() for setcfg

This avoids the issue where, for example, 10.0.0.1/8 is added first,
then 10.0.0.1/8 is deleted, and 10.0.0.1/24 is added.  Netlink changes
observed at the wrong time would make it seem as though IPv4 had
been lost when in fact everthing was fine.

Bug: 26991160
Change-Id: If0dad2993f818686ef8a99618e779813e9c7af90