Break the fs package up into smaller parts.

The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.

The new code layout is documented in CONTRIBUTING.

lib/readers/readfill.go

@@ -0,0 +1,18 @@
+package readers
+
+import "io"
+
+// ReadFill reads as much data from r into buf as it can
+//
+// It reads until the buffer is full or r.Read returned an error.
+//
+// This is io.ReadFull but when you just want as much data as
+// possible, not an exact size of block.
+func ReadFill(r io.Reader, buf []byte) (n int, err error) {
+	var nn int
+	for n < len(buf) && err == nil {
+		nn, err = r.Read(buf[n:])
+		n += nn
+	}
+	return n, err
+}