In order to make sender of a descriptor (unix socket) be restored
_after_ the descriptor in question, we need to find out those
sockets early (before post-prep calls).

The problem is that current code gives us info about who's the
queuer for who only in post-prep hooks, so the peer resolution
should happen right in collect callback.

At the same time we need to make sure that all peers configured
in the image exist, as well as need to put master/slave flags
for socketpairs. Both these actions can only happen in post-prep.

Said that -- the current peer resolve routine should be split
into two steps.

v2: Handle the socket connected to self (dgram)
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>