new command: 'list' to retreive the list of active call-ids
list command: several fixes + documentation

  * duplicate strings when filling the bencode structure
  * limit the number of results to avoid going over the UDP packet
    size limit
Merge branch 'co_list_calls' of github.com:camilleoudot/rtpengine into camilleoudot-co_list_calls
Merge branch 'camilleoudot-co_list_calls'