ifconfig: simplify carp vhid setup.

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.

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D40160
MFC after: 2 weeks