iproute: support families in get_links()

Use case: dump all the bridge ports::

    In [52]: ip.get_links(family=AF_BRIDGE)
    Out[52]:
    [{'__align': 0,
      'attrs': [['IFLA_IFNAME', 'virbr0-nic'],
       ['IFLA_MASTER', 4],
       ['IFLA_MTU', 1500],
       ['IFLA_OPERSTATE', 'DOWN'],
       ['IFLA_ADDRESS', '52:54:00:17:a9:13']],
      'change': 0,
      'event': 'RTM_NEWLINK',
      'family': 7,
      'flags': 4098,
      'ifi_type': 1,
      'index': 5},
     {'__align': 0,
      'attrs': [['IFLA_IFNAME', 'vnet0'],
       ['IFLA_MASTER', 4],
       ['IFLA_MTU', 1500],
       ['IFLA_OPERSTATE', 'UNKNOWN'],
       ['IFLA_ADDRESS', 'fe:54:00:91:f7:e6']],
      'change': 0,
      'event': 'RTM_NEWLINK',
      'family': 7,
      'flags': 69699,
      'ifi_type': 1,
      'index': 11}...