tipc: add framework for node capabilities exchange

The TIPC protocol spec has defined a 13 bit capability bitmap in
the neighbor discovery header, as a means to maintain compatibility
between different code and protocol generations. Until now this field
has been unused.

We now introduce the basic framework for exchanging capabilities
between nodes at first contact. After exchange, a peer node's
capabilities are stored as a 16 bit bitmap in struct tipc_node.

Reviewed-by: Erik Hugne <erik.hugne@ericsson.com>
Reviewed-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>