Make tramp-archive fit for older Emacsen

Make tramp-archive fit for older Emacsen

* lisp/net/tramp-archive.el (tramp-archive-enabled)
(tramp-archive-file-name-handler-alist)
(tramp-archive-file-name-handler): Adapt docstring.
(tramp-register-archive-file-name-handler): Remove it from
`after-init-hook' when unloading.
(tramp-archive-gvfs-host): New defsubst.
(tramp-archive-dissect-file-name): Use it.

* lisp/net/tramp-cmds.el (tramp-cleanup-all-connections):
Check that `tramp-archive-enabled' is bound.

* test/lisp/net/tramp-archive-tests.el (tramp-archive-test42-auto-load):
Check also that tramp-archive is not loaded when Tramp is loaded.
(tramp-archive-test42-delay-load): Adapt test messages.