[NaCl SDK] nacl_io: Disallow nested calls to ki_init/ki_uninit.

Except for test code which needs to be able to temporarily
inject a test KernelProxy and then restore the previous
one after its done.

BUG=
R=binji@chromium.org

Review URL: https://codereview.chromium.org/134883004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252642 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252642