io/ioutil: clean-up of ReadAll and ReadFile
io/ioutil: clean-up of ReadAll and ReadFile

Make ReadAll use the same mechanism as ReadFile.

R=r
CC=golang-dev
http://codereview.appspot.com/4279041

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go