This reacts to confusing behavior of 'auto-save-visited-file-name',
cf. Bug#25478.
* lisp/files.el (auto-save-visited-interval): New customization option.
(auto-save-visited-mode): New global minor mode.
(auto-save-visited-file-name): Make obsolete.
(auto-save--timer): New internal helper variable.
* doc/emacs/files.texi (Auto Save Files): Document
'auto-save-visited-mode' instead of obsolete
'auto-save-visited-file-name'.
(Auto Save Control): Document customization option
'auto-save-visited-interval'.
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