route(8): print nexthop weight in monitor

[pouria]

Summary

Not showing the weight of nexthops in route monitor
is pointless.

I was testing the -weight option in routing subsystem and found out
there is no utility that shows the nexthops of specific route.
After reading the route(8) source code, I found out that route monitor
shows every nexthop of a specific route that is changing.
However, couldn't find their nexthop weight.

This change is also useful for reusing the print_nlmsg_route_nhop for future
implementations.

Test Plan

simply run route monitor and then
add a route with two different gateways.
Result:

# route -n6 monitor
14:33:02.019 PID 1686 add/repl route 3fff::1/128 gw ::1 iface lo0 mtu 16384 weight 1 table inet6.0
						 gw 2001:db8::1 iface vtnet0 mtu 1500 weight 2

Routes with a single nexthop shows weight 0, which is expected for now.