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.

fs/deletemode.go

@@ -0,0 +1,14 @@
+package fs
+
+// DeleteMode describes the possible delete modes in the config
+type DeleteMode byte
+
+// DeleteMode constants
+const (
+	DeleteModeOff DeleteMode = iota
+	DeleteModeBefore
+	DeleteModeDuring
+	DeleteModeAfter
+	DeleteModeOnly
+	DeleteModeDefault = DeleteModeAfter
+)