Add IF_DEBUG_LEVEL() macro to ensure all debug output preparation
is run only if the current debug level is sufficient. Consistently
use it within routing subsystem.
MFC after: 2 weeks
Convert the last remaining pieces of old-style debug messages
to the new debugging framework.
Differential Revision: https://reviews.freebsd.org/D35994
MFC after: 2 weeks
Use unified guidelines for the severity across the routing subsystem.
Update severity for some of the already-used messages to adhere the
guidelines.
Convert rtsock logging to the new FIB_ reporting format.
MFC after: 2 weeks