network: introduce a interface named lxc_netdev_isup().

network: introduce a interface named lxc_netdev_isup().

When we need to know some info about a netdev, such as is_up or not,
we need to read the flag for the netdev.

This patch introduce a interface function named lxc_netdev_isup()
to check is a netdev up or down.

And introduce a network private function named netdev_get_flag()
to get flag for netdev by netlink.

Changelog: 10/15/2015: Return failure if name==NULL to avoid later strlen fun

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>