Feature/fix child process type checking

https://github.com/joyent/node/pull/8392 introduced a test that doesn't fail when the code it is testing is deleted.

Also, behaviour of execFile and spawn have drifted apart again, unnecessarily, I think. This brings them into line again.

/to @cjihrig  
Sounds like a fine idea to me. I don't have the ultimate say, but I think it makes sense.