Most of the pieces has already been described in the previous patches :)
so here's the summary.

* Dump:

When receiving a message, also receive any SCM-s (already there) and when
SCM_RIGHTs one is met -- go ahead and just dump received descriptors using
regular code, but taking current as the victim task.

Few words about file paths resolution -- since we do dump path-ed files
by receiving them from victim's parasite, such files sent via sockets
should still work OK, as we still receive them, just from another socket.

Several problems here:

1. Unix sockets sent via unix sockets form knots. Not supported.
2. Eventpolls sent via unix might themseves poll unix sockets. Knots
   again. Not supported either.

* Restore:

On restore we need to make unix socket wait for the soon-to-be-scm-sent
descriptors to get restored, so we need to find them, then put a dependency.
After that, the fake fdinfo entry is attached to the respective file
descs, when sent the respective descriptors are closed.

https://github.com/xemul/criu/issues/251

v2: Addressed comments from Kirill

* Moved prepare_scms before adding fake fles (with comment)
* Add scm-only fles as fake, thus removing close_scm_fds
* Try hard finding any suitable fle to use as scm one when
  queuing them for unix socket scm list, only allocate a new
  one if really needed
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>