net/route: properly brace the RT_LOG() macro

sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

routing: allow logging framework to be used outside of the subsystem

MFC after: 2 weeks

routing: improve multiline debug

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

routing: convert remnants of DPRINTF to FIB_CTL_LOG().

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

routing: add debug printing helpers for rtentry and RTM* cmds.

MFC after: 2 weeks

routing: fix debug headers added in 6fa8ed43ee0c.

- move debug headers out of COMPAT_FREEBSD32 in rtsock.c
- remove accidentally-added LOG_ defines from syslog.h

MFC after: 2 weeks

routing: improve debugging.

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

routing: improve debug logging

Use standard logging (FIB_XX_LOG) across nhg code instead of using
old-style DPRINTFs.
Add debug object printer for nhgs (`nhgrp_print_buf`).

Example:

```
Jun 19 20:17:09 devel2 kernel: [nhgrp] inet.0 nhgrp_ctl_alloc_default: multipath init done
Jun 19 20:17:09 devel2 kernel: [nhg_ctl] inet.0 alloc_nhgrp: num_nhops: 2, compiled_nhop: 2

Jun 19 20:17:26 devel2 kernel: [nhg_ctl] inet.0 alloc_nhgrp: num_nhops: 3, compiled_nhop: 3
Jun 19 20:17:26 devel2 kernel: [nhg_ctl] inet.0 destroy_nhgrp: destroying nhg#0/sz=2:[#6:1,#5:1]
```

Differential Revision: https://reviews.freebsd.org/D35525
MFC after: 2 weeks

routing: Add unified level-based logging support for the routing subsystem.

Summary: MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D33664