tipc: add link broadcast get

tipc: add link broadcast get

The command prints the actually method that multicast
is running in the system.
Also 'ratio' value for AUTOSELECT method.

A sample usage is shown below:
$tipc link get broadcast
BROADCAST

$tipc link get broadcast
AUTOSELECT ratio:30%

$tipc link get broadcast -j -p
[ {
        "method": "AUTOSELECT"
    },{
        "ratio": 30
    } ]

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>