zebra: Read in nexthop dependencies from the kernel

zebra: Read in nexthop dependencies from the kernel

Add functionality to read in a group from the kernel,
create a hash entry for it, and add its nexthops to
its dependency list.

Further, we create its nhg struct separtely from this,
copying over any nexthops it should reference directly
into it.

Thus, we have two types for representation of the nexthop group:
	nhe->nhg_depends->[nhe, nhe, nhe]

	nhe->nhg->nexthop->nexthop->nexthop

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