syscall: accept pre-existing directories in nacl zip file
syscall: accept pre-existing directories in nacl zip file

NaCl creates /tmp. This lets the zip file populate it.

LGTM=adg
R=adg
CC=golang-codereviews
https://codereview.appspot.com/159600043

wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old