bridge: vlan: add support to show global vlan options

bridge: vlan: add support to show global vlan options

Add support for new bridge vlan command grouping called global which
operates on global options. The first command it supports is "show".
To do that we update print_vlan_rtm to recognize the global vlan options
attribute and parse it properly.
Man page and help are also updated with the new command.

Syntax is: $ bridge vlan global show [ vid VID ] [ dev DEV ]

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>