lib: add support for new redistribute table-direct element

lib: add support for new redistribute table-direct element

Add a new kind of redistributed route that helps to import entries
that are not copied in the default routing table. Contrary to the
'redistribute table' feature, the entries are directly obtained from
the zebra appropriate routing table.

The 'table-direct' naming expresses the direct redistribution of
the routes, without having to copy the route entries in the default
routing table.

The distance value for this route is 14. Such route entries will
be prioritary compared to 'table' and 'ebgp' route entries type.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>