sk-unix: Fix name resolving on nested mount points

In case if socket's cwd lays on nested mount point
we might resolve its path a bit incorrectly
(mount_resolve_path helper should not obtain
 paths with leading dot).

Thus send a path without leading dot for correct
name resolving.

Also add some error messages.

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