tipc: JSON support for showing nametable

tipc: JSON support for showing nametable

Add json output support for nametable show

Example output:
$tipc -j -p nametable show

[ {
        "type": 0,
        "lower": 16781313,
        "upper": 16781313,
        "scope": "zone",
        "port": 0,
        "node": ""
    },{
        "type": 0,
        "lower": 16781416,
        "upper": 16781416,
        "scope": "cluster",
        "port": 0,
        "node": ""
    } ]

v2:
    Replace variable 'json_flag' by 'json' declared in include/utils.h
    Add new parameter '-pretty' to support pretty output

v3:
    Update manual page

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>