enable WARNS_WERROR by default

enable WARNS_WERROR by default

This is to catch new warnings sneaking into buildworld.  WARNS_WERROR
means that -Werror is added to sources iff compiled with WARNS >= 1.
All these sources have been fixed to currently compile without warnings.
Setting WARNS_WERROR helps us to maintain this status.

If your build should fail with a warning treated as error, you either
have introduced a new warning - in which case you will have to fix it
before committing - or you are using CFLAGS that exhibit warnings which
have not been fixed yet - in that case you should try and fix the
warning nevertheless.

*Only* if you are using compile flags which produce excess warnings, you
should disable WARNS_WERROR by setting NO_WERROR in make.conf.  If you
do, make *absolutely* sure that you don't introduce new warnings into
the build, since it will break the build for everybody else who does not
set NO_WERROR!

Discussed-with: swildner@