tipc: add link broadcast set method and ratio

tipc: add link broadcast set method and ratio

The command added here makes it possible to forcibly configure the
broadcast link to use either broadcast or replicast, in addition to
the already existing auto selection algorithm.

A sample usage is shown below:
$tipc link set broadcast BROADCAST
$tipc link set broadcast AUTOSELECT ratio 25

$tipc link set broadcast -h
Usage: tipc link set broadcast PROPERTY

PROPERTIES
 BROADCAST         - Forces all multicast traffic to be
                     transmitted via broadcast only,
                     irrespective of cluster size and number
                     of destinations

 REPLICAST         - Forces all multicast traffic to be
                     transmitted via replicast only,
                     irrespective of cluster size and number
                     of destinations

 AUTOSELECT        - Auto switching to broadcast or replicast
                     depending on cluster size and destination
                     node number

 ratio SIZE        - Set the AUTOSELECT criteria, percentage of
                     destination nodes vs cluster size

Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: Hoang Le <hoang.h.le@dektech.com.au>
Signed-off-by: David Ahern <dsahern@gmail.com>