Add TIPC_NL_BEARER_SET command to the new tipc netlink API.

This command can set one or more link properties for a particular
bearer.

Netlink logical layout of bearer set message:
-> bearer
    -> name
    -> link properties
        [ -> tolerance ]
        [ -> priority ]
        [ -> window ]
Signed-off-by: Richard Alpe <richard.alpe@ericsson.com>
Reviewed-by: Erik Hugne <erik.hugne@ericsson.com>
Reviewed-by: Jon Maloy <jon.maloy@ericsson.com>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>