routing: Add unified level-based logging support for the routing subsystem.
Summary: MFC after: 2 weeks
Approved by: re(gjb)
Differential Revision: https://reviews.freebsd.org/D33664
(cherry picked from commit 63f7f3921bdc468a9b564a530f63480cc82ebd7c)
(cherry picked from commit eb22b73358784341115c83b81eba5303789484b0)