Add the optional keyword "vid" to bridge vlan show so the user can
request filtering by a specific vlan id. Currently the filtering is
implemented only in user-space. The argument name has been chosen to
match the add/del one - "vid". This filtering can be used also with the
"-compressvlans" option to see in which range is a vlan (if in any).
Also this will be used to show only specific per-vlan statistics later
when support is added to the kernel for it.

Examples:
$ bridge vlan show vid 450
port	vlan ids
eth2	 450

$ bridge -c vlan show vid 450
port	vlan ids
eth2	 400-500

$ bridge vlan show vid 1
port	vlan ids
eth1	 1 PVID Egress Untagged
eth2	 1 PVID
br0	 1 PVID Egress Untagged
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>