pppoe: T1318: validate existing source-interface

It is not only sufficient to check if there is a source-interface configured,
but rather it must also be checked if the source-interface exists at all in the
system.

If the interface does not exist pppd will complain with:
pppd[2778]: /usr/sbin/pppd: In file /etc/ppp/peers/pppoe1: unrecognized option 'eth0.202'