interfaces: query permanent MAC address through ethtool

interfaces: query permanent MAC address through ethtool

This is possible since 2.6.14. No need to parse a file. We still need to
do that as root as this is only allowed for mere mortals since
2.6.19 (and we would like to support 2.6.18). As a side note, it's
possible to query settings since 2.6.36 (so priv_ethtool is still
needed).