sbin/ifconfig: Use a global libifconfig handle

This should eventually replace the socket passed to the various
handlers. In the meantime, making it global avoids repeatedly opening
and closing handles.

Reported by: kp
Reviewed by: kp (earlier version)
Differential Revision: https://reviews.freebsd.org/D28990

(cherry picked from commit 8b22242550c98d0a59b6589f2be0beb1dc2eac13)