sk-unix: Don't affect cwd for relative named sockets

When we restore sockets with relative names we change
current working directory into the one provided by
socket image data. This actually affects current
criu state because the rest of code doesn't know
about such tricks and may rely on working dir
consistency.

So remember the current working dir and restore it
back once socket cwd operations are complete.

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