enable DSN (Delivery Status Notification) for the implicit socket too

enable DSN (Delivery Status Notification) for the implicit socket too

DSN is implicitly enabled when using `listen on sock' but it's not for
the implicit socket, avoid this incoherence by enabling it on the
implicit socket too.

Report and diff by Tassilo Philipp (tphilipp at potion-studios dot com)
ok millert@