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
```
