lldpctl: new JSON output

lldpctl: new JSON output

JSON output is done with "Jansson", a convenient JSON library. The
output may be a bit difficult to use when a multivalued field with
only one value is present. In this case, it is not put into an
array. For example, if there is only one neighbor, you get:
`{interface:{eth0: ...}}` while you will get this for two neighbors:
`{interface:[{eth0:...},{eth1:...}]}`.