Use expand-file-name rather than concat.

Use expand-file-name rather than concat.
(doc-view-cache-directory): Add the UID so multiple users won't clash.
(doc-view-current-overlay, doc-view-pending-cache-flush): New vars.
(doc-view-goto-page, doc-view-insert-image, doc-view-buffer-message)
(doc-view-toggle-display): Use an overlay over the whole buffer so as
not to have to touch the buffer's content.
(doc-view-initiate-display): New function, extracted from doc-view-mode.
(doc-view-mode): Use it.  Don't mark as a special mode.
Put the page numbers in the modeline.
Set up the overlay.  Hide the cursor.  Run the mode hook.
Use after-revert-hook rather than revert-buffer-function.
(doc-view-search-internal): Fix typo.
(doc-view-convert-current-doc, doc-view-insert-image): Delay the
image-cache flush.
(doc-view-reconvert-doc): Don't reset the whole mode.
(doc-view-make-safe-dir): New function.
(doc-view-current-cache-dir): Use it.