serve nfs: new 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
```

go.sum

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@@ -436,6 +439,8 @@ github.com/putdotio/go-putio/putio v0.0.0-20200123120452-16d982cac2b8 h1:Y258uzX
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