* doc-view.el: Use (and prefer) soffice as default ODF->PDF

* doc-view.el: Use (and prefer) soffice as default ODF->PDF
converter (Bug#13622).  Patch provided by Jambunathan K
<kjambunathan@gmail.com>.
(doc-view-unoconv-program): Make obsolete variable.
(doc-view-odf->pdf-converter-program): New variable.
(doc-view-odf->pdf-converter-function): New variable.
(doc-view-mode-p): Use it.
(doc-view-odf->pdf-converter-unoconv): Rename from
`doc-view-odf->pdf-converter-unoconv'.
(doc-view-odf->pdf-converter-soffice): New function.
(doc-view-convert-current-doc): Use
`doc-view-odf->pdf-converter-function'.