about: add new command 'about' to get quota info from a remote

Implemented for drive only.

Relates to #1138 and #1564.

fs/fs.go

@@ -377,6 +377,9 @@ type Features struct {
 	// Don't implement this unless you have a more efficient way
 	// of listing recursively that doing a directory traversal.
 	ListR ListRFn
+
+	// Get quota information from the Fs
+	About func() error
 }
 
 // Disable nil's out the named feature.  If it isn't found then it
@@ -466,6 +469,9 @@ func (ft *Features) Fill(f Fs) *Features {
 	if do, ok := f.(ListRer); ok {
 		ft.ListR = do.ListR
 	}
+	if do, ok := f.(Abouter); ok {
+		ft.About = do.About
+	}
 	return ft.DisableList(Config.DisableFeatures)
 }
 
@@ -519,6 +525,9 @@ func (ft *Features) Mask(f Fs) *Features {
 	if mask.ListR == nil {
 		ft.ListR = nil
 	}
+	if mask.About == nil {
+		ft.About = nil
+	}
 	return ft.DisableList(Config.DisableFeatures)
 }
 
@@ -706,6 +715,12 @@ type RangeSeeker interface {
 	RangeSeek(offset int64, whence int, length int64) (int64, error)
 }
 
+// Abouter is an optional interface for Fs
+type Abouter interface {
+	// About gets quota information from the Fs
+	About() error
+}
+
 // ObjectsChan is a channel of Objects
 type ObjectsChan chan Object