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/filesystem.go

@@ -0,0 +1,159 @@
+//go:build unix
+// +build unix
+
+package nfs
+
+import (
+	"os"
+	"path"
+	"strings"
+	"time"
+
+	billy "github.com/go-git/go-billy/v5"
+	"github.com/rclone/rclone/fs"
+	"github.com/rclone/rclone/vfs"
+	"github.com/rclone/rclone/vfs/vfscommon"
+)
+
+// FS is our wrapper around the VFS to properly support billy.Filesystem interface
+type FS struct {
+	vfs *vfs.VFS
+}
+
+// ReadDir implements read dir
+func (f *FS) ReadDir(path string) (dir []os.FileInfo, err error) {
+	return f.vfs.ReadDir(path)
+}
+
+// Create implements creating new files
+func (f *FS) Create(filename string) (billy.File, error) {
+	return f.vfs.Create(filename)
+}
+
+// Open opens a file
+func (f *FS) Open(filename string) (billy.File, error) {
+	return f.vfs.Open(filename)
+}
+
+// OpenFile opens a file
+func (f *FS) OpenFile(filename string, flag int, perm os.FileMode) (billy.File, error) {
+	return f.vfs.OpenFile(filename, flag, perm)
+}
+
+// Stat gets the file stat
+func (f *FS) Stat(filename string) (os.FileInfo, error) {
+	return f.vfs.Stat(filename)
+}
+
+// Rename renames a file
+func (f *FS) Rename(oldpath, newpath string) error {
+	return f.vfs.Rename(oldpath, newpath)
+}
+
+// Remove deletes a file
+func (f *FS) Remove(filename string) error {
+	return f.vfs.Remove(filename)
+}
+
+// Join joins path elements
+func (f *FS) Join(elem ...string) string {
+	return path.Join(elem...)
+}
+
+// TempFile is not implemented
+func (f *FS) TempFile(dir, prefix string) (billy.File, error) {
+	return nil, os.ErrInvalid
+}
+
+// MkdirAll creates a directory and all the ones above it
+// it does not redirect to VFS.MkDirAll because that one doesn't
+// honor the permissions
+func (f *FS) MkdirAll(filename string, perm os.FileMode) error {
+	parts := strings.Split(filename, "/")
+	for i := range parts {
+		current := strings.Join(parts[:i+1], "/")
+		_, err := f.Stat(current)
+		if err == vfs.ENOENT {
+			err = f.vfs.Mkdir(current, perm)
+			if err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+// Lstat gets the stats for symlink
+func (f *FS) Lstat(filename string) (os.FileInfo, error) {
+	return f.vfs.Stat(filename)
+}
+
+// Symlink is not supported over NFS
+func (f *FS) Symlink(target, link string) error {
+	return os.ErrInvalid
+}
+
+// Readlink is not supported
+func (f *FS) Readlink(link string) (string, error) {
+	return "", os.ErrInvalid
+}
+
+// Chmod changes the file modes
+func (f *FS) Chmod(name string, mode os.FileMode) error {
+	file, err := f.vfs.Open(name)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if err := file.Close(); err != nil {
+			fs.Logf(f, "Error while closing file: %e", err)
+		}
+	}()
+	return file.Chmod(mode)
+}
+
+// Lchown changes the owner of symlink
+func (f *FS) Lchown(name string, uid, gid int) error {
+	return f.Chown(name, uid, gid)
+}
+
+// Chown changes owner of the file
+func (f *FS) Chown(name string, uid, gid int) error {
+	file, err := f.vfs.Open(name)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if err := file.Close(); err != nil {
+			fs.Logf(f, "Error while closing file: %e", err)
+		}
+	}()
+	return file.Chown(uid, gid)
+}
+
+// Chtimes changes the acces time and modified time
+func (f *FS) Chtimes(name string, atime time.Time, mtime time.Time) error {
+	return f.vfs.Chtimes(name, atime, mtime)
+}
+
+// Chroot is not supported in VFS
+func (f *FS) Chroot(path string) (billy.Filesystem, error) {
+	return nil, os.ErrInvalid
+}
+
+// Root  returns the root of a VFS
+func (f *FS) Root() string {
+	return f.vfs.Fs().Root()
+}
+
+// Capabilities exports the filesystem capabilities
+func (f *FS) Capabilities() billy.Capability {
+	if f.vfs.Opt.CacheMode == vfscommon.CacheModeOff {
+		return billy.ReadCapability | billy.SeekCapability
+	}
+	return billy.WriteCapability | billy.ReadCapability |
+		billy.ReadAndWriteCapability | billy.SeekCapability | billy.TruncateCapability
+}
+
+// Interface check
+var _ billy.Filesystem = (*FS)(nil)