io/ioutil: use filepath.Join, handle trailing / in $TMPDIR
io/ioutil: use filepath.Join, handle trailing / in $TMPDIR

R=niemeyer
CC=golang-dev
http://codereview.appspot.com/4256057
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go