Add x86_64 to tested platforms.

* Introduce RawFileSystem as direct interface to FUSE kernel layer,
  similar to fuse_lowlevel.h

* Introduce PathFuseFilesystem as path based interface, similar to
  fuse.h.

* Add connector to hook up PathFuseFilesystem with RawFileSystem
  interface.

* Add PassThroughFuse, which exposes os and syscall APIs as a FUSE
  PathFuseFilesystem, for testing.

* Add passthrough_test.go which exercises all of the API.

* Add fuse/README with explicit license notice and some information.

* Remove old 'manager' based API.