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/fserrors/closed_conn_unsupported.go

@@ -0,0 +1,9 @@
+// +build plan9
+
+package fserrors
+
+// isClosedConnErrorPlatform reports whether err is an error from use
+// of a closed network connection using platform specific error codes.
+func isClosedConnErrorPlatform(err error) bool {
+	return false
+}