[NaCl SDK] nacl_io: add placeholder fcntl(2) implementation

Also, add POLLPRI to poll.h.

Both of these are needed to support building libcurl against
nacl_io.

R=binji@chromium.org

Review URL: https://codereview.chromium.org/23654033

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224721 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224721