Changed default for sfwi to enabled, rather than commented.

I've been doing a little git and email archaeology to see if there's a rationale anyplace for no longer shipping with +sfwi #### — I couldn't find one. Given scheduler collapse can still occur on R16 given badly behaved BIFs or long-running NIFs (no dirty_schedulers, that's an R17 feature) we should probably still ship with this configured by default.