general: remove or reword FIXME comments

Some of them don't apply any more (we made our choices, or the problem
has been addressed and fixed elsewhere). Some do but I reworded them.
Some still apply unchanged, and are not touched by this patch.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>