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
The diff is pretty large, but de-facto mechanical. All changes
except the structure setup in ifconfig[_netlink].c are one-line
Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D40239
MFC after: 2 weeks