(doc-view-doc->txt, doc-view-open-text)

(doc-view-doc->txt, doc-view-open-text)
(doc-view-already-converted-p): New functions.
(doc-view-clear-cache): Don't recreate doc-view-cache-directory.
(doc-view-mode-map): Bind C-c C-t to doc-view-open-text.
(doc-view-dvi->pdf-sentinel, doc-view-dvi->pdf)
(doc-view-pdf->txt-sentinel, doc-view-pdf->txt)
(doc-view-ps->pdf-sentinel, doc-view-ps->pdf)
(doc-view-convert-current-doc): Don't hardwire the functions the
sentinels call.  Now they're provided by two args CALLBACK and
CB-ARGS to the functions.
(doc-view-search): Use doc-view-doc->txt.
(doc-view-initiate-display): Use doc-view-already-converted-p.
Mention new binding C-c C-t if doc-view-mode doesn't work.