ifconfig: remove (most of) the usages of global 'struct ifreq ifr'.

It is hard to reason about the contents of 'ifr' at any given time
as nearly every function sets random fields or pointers in this
structure.
Use local on-stack clean 'struct ifreq' for each function instead.

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D40534
MFC after: 2 weeks