iproute2: support device group semantics

iproute2: support device group semantics

Add the group keyword to ip link set, which has the following meaning:
If both a group and a device name are pressent, we change the device's
group to the specified one. If only a group is present, then the
operation specified by the rest of the command should apply on an entire
group, not a single device.

So, to set eth0 to the default group, one would use
	ip link set dev eth0 group default

Conversely, to set all the devices in the default group down, use
	ip link set group default down

Signed-off-by: Vlad Dogaru <ddvlad@rosedu.org>