Fixes #39.

This makes automated import rewriting more robust.

The code confuses the race-detector, and the work-arounds to fool the
race-detector fools humans. No conclusive benchmark data supports
the usefulness of the handle maps.

This is necessary to work around apparent bugs in the OSX FUSE
implementation.

OSXFUSE requires STATFS to always be present.

This protects against deadlock should the kernel issue a lookup
synchronously in response. This behavior has been seen on Linux
3.15.3-200.fc20.x86_64.

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.

This protects against deadlocks if the kernel calls back into
userspace in the notify handler.