Define a new Features() method for Fs

Optional interfaces are becoming more important in rclone,
--track-renames and --backup-dir both rely on them.

Up to this point rclone has used interface upgrades to define optional
behaviour on Fs objects.  However when one Fs object wraps another it
is very difficult for this scheme to work accurately.  rclone has
relied on specific error messages being returned when the interface
isn't supported - this is unsatisfactory because it means you have to
call the interface to see whether it is supported.

This change enables accurate detection of optional interfaces by use
of a Features struct as returned by an obligatory Fs.Features()
method.  The Features struct contains flags and function pointers
which can be tested against nil to see whether they can be used.

As a result crypt and hubic can accurately reflect the capabilities of
the underlying Fs they are wrapping.

dropbox/dropbox.go

@@ -96,8 +96,9 @@ func configHelper(name string) {
 // Fs represents a remote dropbox server
 type Fs struct {
 	name           string           // name of this remote
-	db             *dropbox.Dropbox // the connection to the dropbox server
 	root           string           // the path we are working on
+	features       *fs.Features     // optional features
+	db             *dropbox.Dropbox // the connection to the dropbox server
 	slashRoot      string           // root with "/" prefix, lowercase
 	slashRootSlash string           // root with "/" prefix and postfix, lowercase
 }
@@ -129,6 +130,11 @@ func (f *Fs) String() string {
 	return fmt.Sprintf("Dropbox root '%s'", f.root)
 }
 
+// Features returns the optional features of this Fs
+func (f *Fs) Features() *fs.Features {
+	return f.features
+}
+
 // Makes a new dropbox from the config
 func newDropbox(name string) (*dropbox.Dropbox, error) {
 	db := dropbox.NewDropbox()
@@ -159,6 +165,7 @@ func NewFs(name, root string) (fs.Fs, error) {
 		name: name,
 		db:   db,
 	}
+	f.features = (&fs.Features{CaseInsensitive: true, ReadMimeType: true}).Fill(f)
 	f.setRoot(root)
 
 	// Read the token from the config file