bridge: add support for backup port

bridge: add support for backup port

This patch adds support for the new backup port option that can be set
on a bridge port. If the port's carrier goes down all of the traffic
gets redirected to the configured backup port. We add the following new
arguments:
$ ip link set dev brport type bridge_slave backup_port brport2
$ ip link set dev brport type bridge_slave nobackup_port

$ bridge link set dev brport backup_port brport2
$ bridge link set dev brport nobackup_port

The man pages are updated respectively.
Also 2 minor style adjustments:
- add missing space to bridge man page's state argument
- use lower starting case for vlan_tunnel in ip-link man page (to be
consistent with the rest)

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David Ahern <dsahern@gmail.com>