Send PADT on PPPoE disconnect

Once we've terminated the PPP session, there is no chance of a PPP layer
disconnect. Some PPPoE relays don't detect the PPP session going down, and
depend on a long timeout or a PPPoE PADT to terminate the session.

Send a PADT on disconnect to work around these buggy relays.

Signed-off-by: Simon Farnsworth <simon@farnz.org.uk>