Move struct rtentry definition to nhop_var.h.

One of the goals of the new routing KPI defined in r359823
is to entirely hide`struct rtentry` from the consumers.
It will allow to improve routing subsystem internals and deliver
features much faster.

This is one of the last changes, effectively moving struct rtentry
definition to a net/route_var.h header, internal to the routing subsystem.

Differential Revision: https://reviews.freebsd.org/D24580