MFC r328522:

MFC r328522:

Create deprecation management functions.

gone_in(majar, msg);	If we're running in FreeBSD major, tell
			the user this code may be deleted soon.
			If we're running in FreeBSD major - 1,
			the the user is deprecated and will
			be gone in major.
			Otherwise say nothing.

gone_in_dev(dev, major, msg) Just like gone_in, except use device_printf.

New tunable / sysctl debug.oboslete_panic: 0 - don't panic,
	1 - panic in major or newer , 2 - panic in major - 1 or newer
	default: 0

if NO_OBSOLETE_CODE is defined, then both of these turn into compile
time errors when building for major. Add options NO_OBSOLETE_CODE to
kernel build system.

This lets us tag code that's going away so users know it will be gone,
as well as automatically manage things.

Differential Review: https://reviews.freebsd.org/D13818