(#13396) Unify ifconfig usage and lookup ifconfig path to fix support for recent net-tools

Without this patch applied linux systems with recent net-tools loose
support for ipaddress,macaddress and other network related facts.
This is due to a path change from /sbin/ifconfig to /bin/ifconfig.
This patch fixes this issue by unifying the lookup and execution of
ifconfig and testing where the ifconfig binary is installed.
Merge branch 'master' of github.com:scepticulous/facter