zebra: Add code to get/set interface to pass up from dplane

zebra: Add code to get/set interface to pass up from dplane

1) Add a bunch of get/set functions and associated data
structure in zebra_dplane to allow the setting and retrieval
of interface netlink data up into the master pthread.

2) Add a bit of code to breakup startup into stages.  This is
because FRR currently has a mix of dplane and non dplane interactions
and the code needs to be paused before continuing on.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>