ldpd: reorganize json support and fix a few issues

ldpd: reorganize json support and fix a few issues

* reorganize code to improve maintainability;
* add json output for the l2vpn show commands as well;
* in "show mpls ldp discovery json", nest the adjacencies inside the
  respective interfaces. Also, fix a segfault when trying to show a
  targeted adjacency;
* in "show mpls ldp neighbor", show the address-family of the adjacencies
  of each neighbor. Also, remove "downstreamUnsolicited":true from
  the output;
* in "show mpls ldp interface json", show all interface/address-family
  combinations;

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>