Special kernel config include file for DIAGNOSTIC defaults

[Martin Husemann <martin%duskware.de@localhost>]

Every time we do a release branch, releng is faced with the problem to
disable DIAGNOSTIC on the branch (mostly for benchmark reasons, but this
is a different discussion).

Last time I did this with a minimalistic grep and only hit like 10 files out
of > 130 that may have needed changing.

The same issue arose for netbsd-7 and the discussion can be seen in
ticket #116 (http://releng.netbsd.org/cgi-bin/req-7.cgi?show=116).

As suggested there, I would like to introduce some "properly" named include
file that basically contains the default values for a DIAGNOSTIC (and maybe
in the future other options that we want to toggle between -current and
release branches).

Assuming something like: src/sys/conf/diagnostic.conf with a single line

no options DIAGNOSTIC

in it on release branches, and

options DIAGNOSTIC

in -current.

Everyone is then supposed to remove the hard coded "options DIAGNOSTIC" from
all kernel config they hit and can build-test and replace them with an
include of the new file. If we miss some kernel configs in the initial pass,
we can fix them later - basically noboy complained about wrong DIAGNOSTIC
on netbsd-6 so far.

We would switch it in current, fix all fallout (if any), pullup
the fixes and the toggle, then switch it back on in -current. This makes
releng's life a lot easier.

Opinions?

Martin