…Handle

Name() method was originally left out and defaulted to the base
class which always returns empty. This trigerred incorrect behavior
in serve nfs where it relied on the Name() of the interafce to figure
out what file it was modifying.

This method is copied from RWFileHandle struct.

Added extra assert in the tests.

A subtle bug where dir modification time is not updated when the dir already exists
in the cache. It is only noticeable when some clients use dir modification time to
invalidate cache.

…y.File

billy defines a common file system interface that is used in multiple go packages.
vfs.Handle implements billy.File mostly, only two methods needed to be added to
make it compliant.

An interface check is added as well.

This is a preliminary work for adding serve nfs command.

Summary:
Adding a new command to serve any remote over NFS. This is only useful for new macOS versions where FUSE mounts are not available.
 * Added willscot/go-nfs dependency and updated go.mod and go.sum

Test Plan:
```
go run rclone.go serve nfs --http-url https://beta.rclone.org :http:
```

Test that it is serving correctly by mounting the NFS directory.

```
mkdir nfs-test
mount -oport=58654,mountport=58654 localhost: nfs-test
```

Then we can list the mounted directory to see it is working.
```
ls nfs-test
```

…ut FUSE

Summary:
In cases where cmount is not available in macOS, we alias nfsmount to mount command and transparently start the NFS server and mount it to the target dir.

The NFS server is started on localhost on a random port so it is reasonably secure.

Test Plan:
```
go run rclone.go mount --http-url https://beta.rclone.org :http: nfs-test
```

Added mount tests:
```
go test ./cmd/nfsmount
```