add DIAGNOSTIC back to GENERIC/INSTALL

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

Hello,
for the second time (at last) a kernel issue raised the question of
adding back 'options DIAGNOSTIC' to GENERIC/INSTALL kernels in HEAD.
Several people agreed tha this would be a good thing.

So here's the formal question: would someone object if I add back
'options DIAGNOSTIC' to i386 and amd64 GENERIC and INSTALL kernels,
with a comment saying this should be disabled on release branch
(it would be up to releng to comment it out as part of the release process) ?

I know that DIAGNOSTIC was commented out so that someone could install
a HEAD snapshot and run benchmarks out of the box, but as a side effect
a lot of but are left hidden and only show up when someone tries
to run a Xen kernel (which still have DIAGNOSTIC). See kern/45051 for
another one.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--