Differential Revision:	https://reviews.freebsd.org/D27732

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

Prefer SRCTOP over CURDIR/../../contrib, etc. However, retain the "up
one level" instances of ../ because they are really relative to this
part of the tree and not a means to find the root of the tree. As
such, it's better to leave them since that further the goal of being
able to move directories if watned to in the future.

Differential Revision:	https://reviews.freebsd.org/D9932
Sponsored by:		Netflix
Silence	On:		arch@ (twice)

preparation for 8.0-RELEASE.  Add the previous version of those
libraries to ObsoleteFiles.inc and bump __FreeBSD_Version.

Reviewed by:    kib
Approved by:    re (rwatson)

Reported by: 	     jhb
Discussed with:	     deischen, des, doubg, harti
Approved by:	     re (kensmith)

been bumped since RELENG_5.

Reviewed by:	ru
Approved by:	re (not needed for commit check but in principle...)