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/cache/cache.go

@@ -98,14 +98,14 @@ changed, any downloaded chunks will be invalid and cache-chunk-path
 will need to be cleared or unexpected EOF errors will occur.`,
 			Default: DefCacheChunkSize,
 			Examples: []fs.OptionExample{{
-				Value: "1m",
-				Help:  "1MB",
+				Value: "1M",
+				Help:  "1 MiB",
 			}, {
 				Value: "5M",
-				Help:  "5 MB",
+				Help:  "5 MiB",
 			}, {
 				Value: "10M",
-				Help:  "10 MB",
+				Help:  "10 MiB",
 			}},
 		}, {
 			Name: "info_age",
@@ -132,13 +132,13 @@ oldest chunks until it goes under this value.`,
 			Default: DefCacheTotalChunkSize,
 			Examples: []fs.OptionExample{{
 				Value: "500M",
-				Help:  "500 MB",
+				Help:  "500 MiB",
 			}, {
 				Value: "1G",
-				Help:  "1 GB",
+				Help:  "1 GiB",
 			}, {
 				Value: "10G",
-				Help:  "10 GB",
+				Help:  "10 GiB",
 			}},
 		}, {
 			Name:     "db_path",