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
```

cmd/serve/nfs/server.go

@@ -0,0 +1,61 @@
+//go:build unix
+// +build unix
+
+package nfs
+
+import (
+	"context"
+	"net"
+
+	nfs "github.com/willscott/go-nfs"
+
+	"github.com/rclone/rclone/fs"
+	"github.com/rclone/rclone/vfs"
+	"github.com/rclone/rclone/vfs/vfscommon"
+)
+
+// Server contains everything to run the Server
+type Server struct {
+	opt      Options
+	handler  nfs.Handler
+	ctx      context.Context // for global config
+	listener net.Listener
+}
+
+// NewServer creates a new server
+func NewServer(ctx context.Context, vfs *vfs.VFS, opt *Options) (s *Server, err error) {
+	if vfs.Opt.CacheMode == vfscommon.CacheModeOff {
+		fs.LogPrintf(fs.LogLevelWarning, ctx, "NFS writes don't work without a cache, the filesystem will be served read-only")
+	}
+	// Our NFS server doesn't have any authentication, we run it on localhost and random port by default
+	if opt.ListenAddr == "" {
+		opt.ListenAddr = "localhost:"
+	}
+
+	s = &Server{
+		ctx: ctx,
+		opt: *opt,
+	}
+	s.handler = newHandler(vfs)
+	s.listener, err = net.Listen("tcp", s.opt.ListenAddr)
+	if err != nil {
+		fs.Errorf(nil, "NFS server failed to listen: %v\n", err)
+	}
+	return
+}
+
+// Addr returns the listening address of the server
+func (s *Server) Addr() net.Addr {
+	return s.listener.Addr()
+}
+
+// Shutdown stops the server
+func (s *Server) Shutdown() error {
+	return s.listener.Close()
+}
+
+// Serve starts the server
+func (s *Server) Serve() (err error) {
+	fs.Logf(nil, "NFS Server running at %s\n", s.listener.Addr())
+	return nfs.Serve(s.listener, s.handler)
+}