iproute2: show counter of carrier on<->off transitions

iproute2: show counter of carrier on<->off transitions

This patch allows to display the current counter of carrier on<->off
transitions (IFLA_CARRIER_CHANGES, see kernel commit "expose number of
carrier on/off changes"):

  ip -s -s link show dev eth0
  32: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 ...
    link/ether ................. brd ff:ff:ff:ff:ff:ff
    RX: bytes  packets  errors  dropped overrun mcast
    125552461  258881   0       0       0       10150
    RX errors: length  crc     frame   fifo    missed
               0        0       0       0       0
    TX: bytes  packets  errors  dropped carrier collsns
    40426119   224444   0       0       0       0
    TX errors: aborted fifo    window  heartbeat transns
               0        0       0       0        3

Tested:
  - kernel with patch "net-sysfs: expose number of carrier on/off
    changes": see "transns" column above
  - kernel wthout the patch: "transns" not displayed (as expected)

Signed-off-by: David Decotigny <decot@googlers.com>