ppp: Detailed last error support

Enables last error support for the PPP protocol handlers.
In generic teardown the PPP daemon exit code is translated into
a self explaining error string which is set as interface error
by proto_notify_error in case of failure.

Signed-off-by: Johan Peeters <johan.peeters111@gmail.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

SVN-Revision: 45333