Add renaming of remote buffer file names to Tramp

Add renaming of remote buffer file names to Tramp

* doc/misc/tramp.texi (Default User): Fix typo.
(Cleanup remote connections): Adapt arguments of
`tramp-cleanup-connection'.
(Renaming remote files): New node.
(Frequently Asked Questions): New item "How to save files when a
remote host isn't reachable anymore?".

* etc/NEWS: Add `tramp-rename-files' and `tramp-rename-these-files'.

* lisp/net/tramp-cmds.el (tramp-default-rename-alist)
(tramp-confirm-rename-file-names): New defcustoms.
(tramp-rename-read-file-name-dir)
(tramp-rename-read-file-name-init): New defsubsts.
(tramp-default-rename-file, tramp-rename-files)
(tramp-rename-these-files): New defuns.

* lisp/net/tramp-integration.el (ido, ivy): Integrate with them.