fix recent net-tools support and unify ifconfig usage ( fixes bug #13396 )

( this is an improved reversion of pull request #385 )

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.