bridge: Support the new bridging attributes

This provides support for the new bridging attributes provided
in IFLA_PROTINFO while maintaining backwards compatibility
with older kernels.

A set of new API functions are exported to access the bridging
information. rtnl_link_bridge_has_ext_info() can be used to
check whether a bridge object has been constructed based on
the newly available attributes or the old message format.

Signed-off-by: Thomas Graf <tgraf@suug.ch>