Add opposites for commandline flags

As a general principle, every commandline flag should have a corresponding opposite so that it's possible to override it in a later part of the argv. Another use case is ghci (e.g. undo the effect of :set -ddump-splices; currently :unset doesn't work).

At the moment, this is missing for many flags (-debug, -ddump-cmm, -dcore-lint, -dno-typeable-binds etc.). This ticket is about performing a systematic review.