isisd: move spf datastructures to a header, to share with fabricd

isisd: move spf datastructures to a header, to share with fabricd

By moving the spf datastructures to a header, fabricd can access the
results of the spf run for flooding optimization or fabric locality
calculation.

While this was deemed a sensible choice in this case, when compared with
the option of adding a lot of OpenFabric specific code to isis_spf.c,
the datastructures should still not be accessed randomly all over the
code base. To make this more clear, the new header was called
isis_spf_private.h (Think of a friend class)

Signed-off-by: Christian Franke <chris@opensourcerouting.org>