bgpd, lib, pimd: Make nexthop_update decoding common

bgpd, lib, pimd: Make nexthop_update decoding common

Create a zapi_nexthop_update_decode function that both
pim and bgp use to decode the message from zebra.

There probably could be further optimizations but I opted
to keep the code as similiar as is possible between the
originals because they both make some assumptions about
code flow that I do not fully understand yet.

The real goal here is that I want to create a new
user of the nexthop tracking code from a higher level
daemon and I see no need to re-implement this damn
code again for a 3rd time.

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