Use binary prefixes for size and rate units

Includes adding support for additional size input suffix Mi and MiB, treated equivalent to M.
Extends binary suffix output with letter i, e.g. Ki and Mi.
Centralizes creation of bit/byte unit strings.

backend/koofr/koofr.go

@@ -534,7 +534,7 @@ func (f *Fs) DirMove(ctx context.Context, src fs.Fs, srcRemote, dstRemote string
 	return nil
 }
 
-// About reports space usage (with a MB precision)
+// About reports space usage (with a MiB precision)
 func (f *Fs) About(ctx context.Context) (*fs.Usage, error) {
 	mount, err := f.client.MountsDetails(f.mountID)
 	if err != nil {