Provides more OSX-specific kernel messages.

CreateIn, MknodIn, SetXAttrIn, and GetXAttrIn are separated into Linux and
Darwin-specific versions.

Tested on OSX 10.9.3 with OSXFuse 2.6.4 and Go 1.2.1.

OpenIn is updated to match Linux and OSXFuse definitions - changing the Unused
field to Mode.