Remove all cb-args, use closures instead.

Remove all cb-args, use closures instead.
(doc-view-sentinel): Merge doc-view-dvi->pdf-sentinel,
doc-view-ps->pdf-sentinel, and doc-view-pdf->txt-sentinel (which was
doing an incorrect check).  Update all callers to use the new name.
(doc-view-doc->txt): Add missing `txt' argument.