[RFC] system() prefer pipes over tmpfiles

As mentioned by @tarruda, it's a bit nasty that `system()` has to use tempfiles, we can remove that. This is a proof of concept for that.

@Shougo, if you feel up for it, you can benchmark this branch to see if this gains us anything. I might have to turn off the debugging messages though, they might be slowing everything down (I suspect so).

ref #473