Revert "ppp-manager: don't use kernel pppoe (rh #1034860, rh #1061641)"

Revert "ppp-manager: don't use kernel pppoe (rh #1034860, rh #1061641)"

We switched to user-mode PPPoE client to work around a bug in kernel
PPPoE code that caused pppd not to be notified when the connection was
terminated with a PADT before the LCP Termination Request.

The kernel bug has now been fixed upstream with commit 287f3a943fef
"pppoe: Use workqueue to die properly when a PADT is received", queued
for v4.1.

Since the issue affected only very particular scenarios and could
somehow be solved with the right configuration (see bugzilla entry),
we can safely revert the patch and restore the use of kernel mode
PPPoE.

This reverts commit 7955806a02db64b20079267743056d7d9d45af3b.

https://bugzilla.gnome.org/show_bug.cgi?id=742939