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/config/config_read_password_unsupported.go

@@ -0,0 +1,12 @@
+// ReadPassword for OSes which are not supported by golang.org/x/crypto/ssh/terminal
+// See https://github.com/golang/go/issues/14441 - plan9
+//     https://github.com/golang/go/issues/13085 - solaris
+
+// +build solaris plan9
+
+package config
+
+// ReadPassword reads a password with echoing it to the terminal.
+func ReadPassword() string {
+	return ReadLine()
+}