Merge libdispatch-685

Unchanged import from Apple open source release at
http://opensource.apple.com/tarballs/libdispatch/libdispatch-685.tar.gz

move stdio.h include to dispatch.h

Signed-off-by: Daniel A. Steffen <dsteffen@apple.com>

style, whitespace

Signed-off-by: Daniel A. Steffen <dsteffen@apple.com>