[NaCl SDK] nacl_io: Fix open flags for socket handles.

Make sure we always call Init(O_RDWR) on new socket handles
that get created.

R=binji@chromium.org

Review URL: https://codereview.chromium.org/41223002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231021 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231021