sk-unix: Add trivial name resolver for sockets with relative names

Unix sockets may be created with non-absolute (relative) path
(when kernel creates one it always use AT_FDCWD for name resolving),
So when we collect sockets we see them as having names without leading
slash.

In common cases for such sockets application doesn't change own
working directory after that but this is not always the true.
So we need to invent some name resolver. The good candidate is
IRMAP cache but after a number of testings I found that it might
slow down performance very dramatically. Thus we need some more
intelligent way here.

For a while, for common applications such as postfix, fetching
dumpee working directory and root is enough. So here what we do

 - when socket get collected from diag interface we remember
   its relative name parameters (device and inode) but postprone
   name resolving to not bring perf penalty until really needed

 - when we meet a socket to dump with relative name assigned we
   try to use $cwd/name and $root/name for this socket to check
   if it has been created in those directories. On success we
   simply remember the directory in image and when restore such
   socket call for chdir helper to change working dir and generate
   relative name

v2:

 - Use new unlink_stale to remove sockets we're to restore
 - Use *at() helpers once we're changed working dir in bind_unix_sk
 - Add more debug ouput

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>