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