argpar.h: Doxygenize

This patch documents the argpar API with Doxygen, reusing most of the
existing unformatted documentation, and adding more details.

The configuration script generates `Doxyfile` from `Doxyfile.in` to
include the version number of the project.

If you have the Doxygen tool, you can then generate the HTML
documentation as such:

    $ doxygen

The output is `api-doc/html/index.html`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I73e5edf4adb5e0d4af8ecd08417362b9df4d53db