(doc-view-conversion-buffer): Give it an internal name.

(doc-view-conversion-buffer): Give it an internal name.
Demote it to plain defvar.
(doc-view-conversion-refresh-interval): Reduce interval.
(doc-view-goto-page): Allow moving to pages not yet rendered.
(doc-view-goto-page): Construct a file name rather than extracting it
from doc-view-current-files.
(doc-view-kill-proc): Ignore errors from kill-process.
(doc-view-pdf/ps->png-sentinel): Die gracefully if the buffer is dead.
(doc-view-insert-image): Use appropriate text if the page hasn't been
rendered yet.  Adjust scrolling so the text is displayed.
(doc-view-display): Detect not just that a page is available, but also
that it wasn't available before, to avoid refreshing all pages repeatedly.
(doc-view-mode): Make doc-view-cache-directory if needed.