Allow nil FileHandles.

Change DefaultOperations to be for a ReadOnly in-memory filesystem.

The fd is set on creation, and never written. We rely on the kernel
and on fileEntry.wg to synchronize such that RELEASE always happens
after read/write has completed.

This makes the inodes in loopback reflect the underlying inodes if
they don't span mounts

* MutableDirOperations
* XAttrOperations
* LockOperations

add MaxPages to InitOut

Use a []uint32 in Inode to keep track of open files. This reduces the
memory consumption of this feature.

Add a test that tries to (unsuccessfully) trigger file closing on the
while file.GetAttr is under way

This is necessary to provide a file handle to GetAttr.

The protocol is:

  request: read (with buffer 1024)
  response: data (if buffer is big enough), or (data size, ERANGE)

Before, the raw API provided

* GetXAttrSize: called if the kernel sends a 0 sized buffer (which
  never happens

* GetXAttrData: called in other cases. The return value is []byte
  forcing allocation of potentially large buffers

* ListXAttr: return value []byte, so also problematic for allocation.

Now, the raw API mirrors the kernel API more closely, and use recycled
buffers for more efficiency.

This should be tagged with a version bump because it changes the
RawFileSystem API.

Changes to nodefs and paths will be transparent: a Cancel channel was
added to the fuse.Context struct. In addition fuse.Context now
implements the Go context package. This also increases the minimum Go
version to 1.7.

compile on darwin too