pppd: Default exit status to EXIT_CONNECT_FAILED during connection phase

pppd: Default exit status to EXIT_CONNECT_FAILED during connection phase

The rp-pppoe plugin doesn't set the exit status in its connect
function, resulting in pppd exiting with a status of EXIT_OK (0)
if rp-pppoe fails to connect.  This fixes the problem for rp-pppoe
and any other plugins that don't set the exit status explicitly
by making the status default to EXIT_CONNECT_FAILED if the channel's
connect function fails.

Reported-by: Peter Warasin <peter@endian.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>