This patch adds an M_NOFREE flag which allows one to mark an mbuf as
not being independently freeable. This allows one to embed an mbuf in
the cluster itself. This confers the benefits of the packet zone on
all cluster sizes. Embedded mbufs currently suffer from the same
limitation that packet zone mbufs do in that one cannot disconnect
them and pass them around independently of the cluster. It would
likely be possible to eliminate this limitation in the future by
adding a second reference for the mbuf itself.

Approved by: re(gnn)