fuse: Split off node FS code into fuse/nodefs/ package.

Update client code with:

gofmt -w -r 'fuse.File -> nodefs.File' $*
gofmt -w -r 'fuse.StatfsOut -> nodefs.StatfsOut' $*
gofmt -w -r 'fuse.FileSystemConnector -> nodefs.FileSystemConnector' $*
gofmt -w -r 'fuse.Inode -> nodefs.Inode' $*
gofmt -w -r 'fuse.NodeFileSystem -> nodefs.FileSystem' $*
gofmt -w -r 'fuse.FileSystemOptions -> nodefs.Options' $*
gofmt -w -r 'fuse.NewFileSystemOptions -> nodefs.NewOptions' $*
gofmt -w -r 'fuse.FsNode -> nodefs.Node' $*
gofmt -w -r 'fuse.WithFlags -> nodefs.WithFlags' $*
gofmt -w -r 'fuse.NewLoopbackFile -> nodefs.NewLoopbackFile' $*
gofmt -w -r 'x.FsNode -> x.Node' $*
gofmt -w -r 'fuse.NewReadOnlyFile -> nodefs.NewReadOnlyFile' $*
gofmt -w -r 'fuse.MountNodeFileSystem -> nodefs.MountFileSystem' $*
gofmt -w -r 'fuse.NewDataFile -> nodefs.NewDataFile' $*
gofmt -w -r 'fuse.NewDefaultFile -> nodefs.NewDefaultFile' $*
gofmt -w -r 'fuse.NewMemNodeFs -> nodefs.NewMemNodeFs' $*
gofmt -w -r 'fuse.NewFileSystemConnector -> nodefs.NewFileSystemConnector' $*
gofmt -w -r 'fuse.NewDefaultNodeFileSystem -> nodefs.NewDefaultFileSystem' $*
gofmt -w -r 'fuse.NewDefaultFsNode -> nodefs.NewDefaultNode' $*
gofmt -w -r 'fuse.NewDevNullFile -> nodefs.NewDevNullFile' $*
fuse/nodefs: hide MemNodeFs type, document NewMemNodeFs().
Rename MountState to Server and merge New and Mount functionality in NewServer.

gofmt -w -r 'MountState -> Server' $*
gofmt -w -r 'NewMountState -> NewServer' $*
gofmt -w -r 'x.Loop -> x.Serve' $*