Move <add|del|change>_route() functions to route_ctl.c in preparation of
multipath control plane changed described in D24141.

Currently route.c contains core routing init/teardown functions, route table
manipulation functions and various helper functions, resulting in >2KLOC
file in total. This change moves most of the route table manipulation parts
to a dedicated file, simplifying planned multipath changes and making
route.c more manageable.

Differential Revision: https://reviews.freebsd.org/D24870