ox-latex: support :float nil with caption for minted listings

ox-latex: support :float nil with caption for minted listings

* lisp/ox-latex.el (org-latex-src-block): Support :float nil with
caption for minted.
(org-latex-listings): Edit docstring to describe this usecase.
* lisp/org.el (org-latex-default-packages-alist): Add “capt-of”
package.

This takes advantage of the capt-of package’s \captionof command,
which allows to insert a caption (with autogenerated number and
\ref-able label) without creating a floating environment.  One example
of where this is useful is in the case of a minted source code listing
that spans more than one page.  (Latex floats can’t be larger than a
page, generally speaking.)

The listings package already handles this case using its own mechanism.