- StartProcess will work with relative (to attr.Dir, not
  current directory) executable filenames
- StartProcess will only work if executable filename points
  to the real file, it will not search for executable in the
  $PATH list and others (see CreateProcess manual for details)
- StartProcess argv strings can contain any characters

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4306041