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.

s3/s3.go

@@ -216,11 +216,12 @@ var (
 // Fs represents a remote s3 server
 type Fs struct {
 	name               string           // the name of the remote
+	root               string           // root of the bucket - ignore all objects above this
+	features           *fs.Features     // optional features
 	c                  *s3.S3           // the connection to the s3 server
 	ses                *session.Session // the s3 session
 	bucket             string           // the bucket we are working on
 	acl                string           // ACL for new buckets / objects
-	root               string           // root of the bucket - ignore all objects above this
 	locationConstraint string           // location constraint of new buckets
 	sse                string           // the type of server-side encryption
 	storageClass       string           // storage class
@@ -264,6 +265,11 @@ func (f *Fs) String() string {
 	return fmt.Sprintf("S3 bucket %s path %s", f.bucket, f.root)
 }
 
+// Features returns the optional features of this Fs
+func (f *Fs) Features() *fs.Features {
+	return f.features
+}
+
 // Pattern to match a s3 path
 var matcher = regexp.MustCompile(`^([^/]*)(.*)$`)
 
@@ -373,6 +379,7 @@ func NewFs(name, root string) (fs.Fs, error) {
 		sse:                fs.ConfigFileGet(name, "server_side_encryption"),
 		storageClass:       fs.ConfigFileGet(name, "storage_class"),
 	}
+	f.features = (&fs.Features{ReadMimeType: true, WriteMimeType: true}).Fill(f)
 	if *s3ACL != "" {
 		f.acl = *s3ACL
 	}