GetConnectionCredentials - add smack support

GetConnectionCredentials - add smack support

A process should never change its Smack label while connected to
D-Bus. If it did, we would end up with race conditions around
permission checking. Therefore we can retrieve the Smack label once,
when the process connects, and use that label whenever it is needed.

A new public libdbus API also gets added: dbus_connection_get_smack_label()
This is primarily for dbus-daemon, but may also be useful for other applications
creating direct connections.

Change-Id: I16ec50a031809aab879a543ec2d7effd56768bf1