zebra: Re-organize/expose nhg_connected

zebra: Re-organize/expose nhg_connected

Re-organize and expose the nhg_connected functions so that
it can be used outside zebra_nhg.c. And then abstract those
into zebra_nhg_depends_* and zebra_nhg_depenents_* functons.

Switch the ifp struct to use an RB tree for its dependents,
making use of the nhg_connected functions.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>