NetLabel: Introduce static network labels for unlabeled connections

Most trusted OSs, with the exception of Linux, have the ability to specify
static security labels for unlabeled networks.  This patch adds this ability to
the NetLabel packet labeling framework.

If the NetLabel subsystem is called to determine the security attributes of an
incoming packet it first checks to see if any recognized NetLabel packet
labeling protocols are in-use on the packet.  If none can be found then the
unlabled connection table is queried and based on the packets incoming
interface and address it is matched with a security label as configured by the
administrator using the netlabel_tools package.  The matching security label is
returned to the caller just as if the packet was explicitly labeled using a
labeling protocol.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>