Restore of namespaces requires executions of external tools
(ip, tar, etc). We want to know return codes, so we should
block a default sigchld handler. Before we did that for each
command, I suggest to block SIGCHLD, then restore namespace and
unblock SIGCHLD.

The default sigchld handler is used for catching target processes,
but all this processes (except a current one ) are started after
restoring namespaces.

Currently we forgot to block SIGCHLD before executing "ip",
and this bug was caught.
Reported-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>