ifconfig: simplify carp vhid setup.

[melifaro]

Summary

Currently carp implementation peeks into the opaque 'afp->af_addreq' buffer, assumes it knows the af-specific layout and assigns vhid directly.
Simplify the code and remove abstraction leak by introducing per-afp callback for setting vhid.
This change is a pre-requisite to set addresses via Netlink, as Netlink implementiation uses different structure layout.