lib, zebra: Allow for installation of a weighted nexthop

lib, zebra: Allow for installation of a weighted nexthop

Linux has the idea of allowing a weight to be sent
down as part of a nexthop group to allow the kernel
to weight particular nexthop paths a bit more or less
than others.

See:
http://tldp.org/HOWTO/Adv-Routing-HOWTO/lartc.rpdb.multiple-links.html

Allow for installation into the kernel using the weight attribute
associated with the nexthop.

This code is foundational in that it just sets up the ability
to do this, we do not use it yet.  Further commits will
allow for the pass through of this data from upper level protocols.

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