Last part of the rework of handleEvents(), this time we take the
getExternalInterface() function and add it to the handleEvents()
function. That way, waybar can react immediately when a new "external
interface" is available and doesn't need to probe. Also that avoid to
have two different functions consuming from the same socket and we
don't need to recode some of the functions that are already available
via libnl (to send and receive messages).