net/tap: add link status notification

net/tap: add link status notification

As tap is a virtual device, there's no physical way a link can be cut.
However, it has an associated kernel netdevice and possibly a remote
netdevice too. These netdevices link status may change outside of the
DPDK scope, through an external command such as:

  ip link set dev tapX down

This commit implements link status notification through netlink.

Signed-off-by: Pascal Mazon <pascal.mazon@6wind.com>
Acked-by: Keith Wiles <keith.wiles@intel.com>