unix: Fix tail processing for peer-less sockets

We should only get to the post-open stage for standalone
sockets with peers. All the others return 0 from ->open
which means "restore is over".

This change avoids double calling or rst_file_params and
restore_socket_opts for those ones.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>