Make ip utility veth driver aware

Make ip utility veth driver aware

The new command is called "veth" with the following syntax:
* ip veth add <dev1> <dev2>
  creates interconnected pair of veth devices.
* ip veth del <dev>
  destroys the pair of veth devices, where <dev> is either
  <dev1> or <dev2> used to create the pair.

One question that is to be solved is whether or not to create
a hard-coded netlink family for veth driver. Without it the
family resolution code has to be moved to general place in ip
utility (by now it is copy-paste-ed from one file to another
till final decision).

Signed-off-by: Pavel Emelianov <xemul@openvz.org>