Make Tramp backward compatible

Make Tramp backward compatible

* lisp/net/tramp-cmds.el (tramp-change-syntax):
Set tramp-autoload cookie.

* lisp/net/tramp-compat.el: Run `tramp-change-syntax' at
startup, if necessary.

* lisp/net/tramp.el (tramp-syntax): Use `tramp-compat-user-error'.
(tramp-register-autoload-file-name-handlers): Do not mark
`operations' for `tramp-file-name-handler'.
(tramp-register-file-name-handlers): Remove also
`tramp-autoload-file-name-handler' for backward compatibility.
(tramp-register-foreign-file-name-handler): Use `delete-dups'.

* test/lisp/net/tramp-tests.el (tramp-change-syntax): Declare.