Restructure struct prefix a bit and move the rib pointer to the union

Restructure struct prefix a bit and move the rib pointer to the union
that splits the normal RIB linkage vs the adjrib-out linkage. This is
done to make a bit of space to put an extra add-path related id into
the struct without blowing its size over 128 bytes.
Long run this struct should be split up but the necessary changes are
too large right now so this is the 2nd best option.
OK benno@