check return value of rtnl_send and related functions

Use warn_unused_result to enforce checking return value of rtnl_send,
and fix where the errors are.

Suggested by initial patch from Petr Písař <ppisar@redhat.com>
veth: fix uninitialized arguments

Based on patch by Sergey Popovich <popovich_sergei@mail.ru>
This fixes crash when ip-link(8) invoced with command:

  ip link add dev veth1a type veth peer
Handle netdev group for veth peer too

Currently ip-link(8) parses, but ignores "group" argument to
peer interface on veth creation.

Insert IFLA_GROUP attribute for peer interface when present.

Signed-off-by: Sergey Popovich <popovich_sergei@mail.ru>
ip: Add HSR support

Add basic support for High-Availability Seamless Redundancy (HSR) network
devices.

Signed-off-by: Arvid Brodin <arvid.brodin@alten.se>