This feature isn't upstream yet. It's required to test a patch in
my local tree.
Makes the link parser understand IFLA_AF_SPEC and call the address
family specific parser.
Introduces a new API to handle address familiy specific link data such as
IFLA_PROTINFO. It provides entry hooks for parsing IFLA_PROTINFO attributes
and allows to include the parsed data when a link object is dumped.
