Set proper label for notification sockets

Set proper label for notification sockets

When a client registers for notification it receives a socket to wait on. The
socket descriptor is transferred using ancillary data. In such cases Smack
checks if Smack rules allow the process that is about to receive it to write to
socket's IPOUT (System::Privileged) and if socket IPIN is allowed to write the
process. CAP_MAC_OVERRIDE is ignored (this may be a bug in Smack). As a result
any process not having System::Privileged label (including ode-admin-cli and UI
apps) is not able to receive the notification socket.

By default notification sockets receive the server's label that is
System::Privileged. This patch sets the IPOUT socket label to '@' so that all
processes can write it and receive the notification socket.

Change-Id: I473099f48e253c4bfe3cebee1a21857d9ea2b963