[NaCl SDK] add rename(2) support to nacl_io.

Currently only supported by memfs.

R=binji@chromium.org

Review URL: https://codereview.chromium.org/64823004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233947 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233947