build: provide a default WARNS for all in-tree builds

The current default is provided in various Makefile.inc in some top-level
directories and covers a good portion of the tree, but doesn't cover parts
of the build a little deeper (e.g. libcasper).

Provide a default in src.sys.mk and set WARNS to it in bsd.sys.mk if that
variable is defined. This lets us relatively cleanly provide a default WARNS
no matter where you're building in the src tree without breaking things
outside of the tree.

Crunchgen has been updated as a bootstrap tool to work on this change
because it needs r365605 at a minimum to succeed. The cleanup necessary to
successfully walk over this change on WITHOUT_CLEAN builds has been added.

There is a supplemental project to this to list all of the warnings that are
encountered when the environment has WARNS=6 NO_WERROR=yes:
https://warns.kevans.dev -- this project will hopefully eventually go away
in favor of CI doing a much better job than it.

Reviewed by:	emaste, brooks, ngie (all earlier version)
Reviewed by:	emaste, arichardson (depend-cleanup.sh change)
Differential Revision:	https://reviews.freebsd.org/D26455

gnu/usr.bin/diff3/Makefile

@@ -18,6 +18,8 @@ CFLAGS+=-I${SRCTOP}/contrib/diff/lib
 CFLAGS+=-DHAVE_CONFIG_H
 CFLAGS+=-DDEFAULT_DIFF_PROGRAM=\"/usr/bin/diff\"
 
+WARNS?=	0
+
 .for f in diff3.c
 ${f}: ${DIFFSRC}/${f} ${.CURDIR}/${f}.diff
 	patch -s -o ${.TARGET} < ${.CURDIR}/${f}.diff ${DIFFSRC}/${f}