Fix ifconfig parsing

[jsl123]

Fixed some small issues...
BTW: Thank you for the script and the necessary information! :-)

Things done

• Minor markdown fix
• Made MAC detection more robust. ifconfig is only used as fallback, since most of the time we use ip in this script anyway...
• Changed installation folder from /usr/bin/ to /usr/sbin since this script may only be run by root as it does change IP addresses. Linux FHS says to install such scripts in a "root only" folder