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/qingstor/qingstor.go

@@ -80,7 +80,7 @@ func init() {
 			Help: `Cutoff for switching to chunked upload
 
 Any files larger than this will be uploaded in chunks of chunk_size.
-The minimum is 0 and the maximum is 5GB.`,
+The minimum is 0 and the maximum is 5 GiB.`,
 			Default:  defaultUploadCutoff,
 			Advanced: true,
 		}, {