[NetLabel]: documentation

Documentation for the NetLabel system, this includes a basic overview
of how NetLabel works, how LSM developers can integrate it into their
favorite LSM, as well as documentation on the CIPSO related sysctl
variables.  Also, due to the difficulty of finding expired IETF
drafts, I am including the IETF CIPSO draft that is the basis of the
NetLabel CIPSO implementation.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>