Add Doxygen comments to the condvar class. Only the C++ interface
(condvar's methods) is documented, not the alternative C interface
(condvar_* functions).

A reminder: run "doxygen" and point your browser to doxyout/html/index.html
to see the API documentation we have so far.

A lot can still be added to this condvar documentation, including a
good introduction to how to use condition variables, why they have
a mutex, etc. But it's at least a start.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>

This patch begins to document OSv's native (non-Linux-compatible) C++ APIs
using Doxygen. With doxygen, functions, classes, and other APIs are
documented using comments inside the source code, similarly to the way
Javadoc is used in the Java world.

After this patch, running "doxygen" generates the documentation in HTML
form (and also ugly ones in LaTeX...) in the "doxyout" directory.
After running "doxygen" in the OSv top-level directory, open your browser to
file://.../doxyout/html/index.html

 to view this documentation.

This patch adds a doxygen configuration file, "Doxyfile", and documents
three functions in the osv namespace: osv::halt(), osv::poweroff(),
osv::halt().

The format of the resulting documentation is not perfect, but it's not
as bad as I feared, and also the documentation doesn't uglify the code
too much (it mainly impacts the header files), so I now believe it will
better for us than writing man pages (an avenue which I explored
previously).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>