ip monitor: Fix issue when timestamp is printed w/o msg

ip monitor: Fix issue when timestamp is printed w/o msg

The issue was observed when IPv6 router broadcasted NDUSEROPT
messages which are not handled by monitor and caused printing
'Timestamps' w/o message because such kind of rtnl messages is not
handled by monitor.

As 'ip monitor' by default subscribes to the all mcast rtnl groups except
RTGRP_TC then all messages of these rtnl groups which are not handled by
monitor may cause such issues.

Fixed by subscribing by default to rtnl mcast groups which are
supported by 'ip monitor'.

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>