routing: remove info argument from add/change_route_nhop().

Currently, rt_addrinfo(info) serves as a main "transport" moving
state between various functions inside the routing subsystem.
As all of the fields are filled in directly by the customers, it
is problematic to maintain consistency, resulting in repeated checks
inside many functions. Additionally, there are multiple ways of
specifying the same value (RTAX_IFP vs rti_ifp / rti_ifa) and so on.
With the upcoming nhop(9) kpi it is possible to store all of the
required state in the nexthops in the consistent fashion, reducing the
need to use "info" in the KPI calls.
Finally, rt_addrinfo structure format was derived from the rtsock wire
format, which is different from other kernel routing users or netlink.

This cleanup simplifies upcoming nhop(9) kpi and netlink introduction.

Reviewed by: zlei.huang@gmail.com
Differential Revision: https://reviews.freebsd.org/D35972
MFC after: 2 weeks