(doc-view-current-doc): Remove. Replace all uses by buffer-file-name.

(doc-view-current-doc): Remove.  Replace all uses by buffer-file-name.
(doc-view-menu): New menu.
(doc-view-reconvert-doc): Don't bind inhibit-read-only.  Remove unused `doc'.
(doc-view-sort): Simplify.
(doc-view-buffer-message): Don't change buffer-modified-p.
(doc-view-mode): Change it here instead.
Tighten scoping of prev-major-mode.  Don't re-insert the file's content.
Don't modify the global value of revert-buffer-function.