Allow mixed export of numbered and unnumbered sections in LaTeX
* lisp/org-latex.el (org-export-latex-subcontent): Deal specially with
the case that NUM is an integer.
We would sometimes like to have numbered \sections in LaTeX export but
unnumbered \subsections and so forth. That is, use the starred
equivalents for all sectioning commands below a certain level.
Previously, the num: option specification could only specify whether
sections should be numbered or unnumbered at all levels. We now treat
an integer value specially, if num:N is supplied then the highest N
levels are numbered, and lower levels are exported without numbering.