isisd: show isis route uses ttable for paths

isisd: show isis route uses ttable for paths

This is a preliminary commit, so that route paths are visible from json.

Before:
> IS-IS paths to level-1 routers that speak IPv6
> Vertex               Type         Metric Next-Hop             Interface Parent
> rt1
> 2001:db8:1000::1/128 IP6 internal 0                                     rt1(4)
> rt2                  TE-IS        10     rt2                  eth-rt2   rt1(4)
> rt3                  TE-IS        10     rt3                  eth-rt3   rt1(4)
> 2001:db8:1000::2/128 IP6 internal 20     rt2                  eth-rt2   rt2(4)
> 2001:db8:1000::3/128 IP6 internal 20     rt3                  eth-rt3   rt3(4)

After:
> Vertex                Type          Metric  Next-Hop  Interface  Parent
>  -------------------------------------------------------------------------
>  rt1
>  2001:db8:1000::1/128  IP6 internal  0                            rt1(4)
>  rt2                   TE-IS         10      rt2       eth-rt2    rt1(4)
>  rt3                   TE-IS         10      rt3       eth-rt3    rt1(4)
>  2001:db8:1000::2/128  IP6 internal  20      rt2       eth-rt2    rt2(4)
>  2001:db8:1000::3/128  IP6 internal  20      rt3       eth-rt3    rt3(4)

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>