bridge: Provide rtnl_link_bridge_alloc()

Signed-off-by: Thomas Graf <tgraf@suug.ch>
netlink: Forward declare frequent libnl types to ease inclusion deps

Signed-off-by: Thomas Graf <tgraf@suug.ch>
vlan: Provide rtnl_link_vlan_alloc()

Signed-off-by: Thomas Graf <tgraf@suug.ch>
bond: Provide rtnl_link_bond_alloc()

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