ifconfig: introduce `ifconfig_context` to store current global state.

ifconfig: introduce `ifconfig_context` to store current global state.

The structure consists of all current context - arguments,
open sockets, current family and so on.

Pass this structure as a first argument to most of the af_ menthods.
This allows to propagate and update shared data without using
 global variables.

The diff is pretty large, but de-facto mechanical. All changes
 except the structure setup in ifconfig[_netlink].c are one-line
 mechanical changes.

Reviewed by:	kp
Differential Revision: https://reviews.freebsd.org/D40239
MFC after:	2 weeks