add an libao audio backend This backend uses libao to play music. It's a bit convoluted since libao doesn't provide an async interface, so we have to run it in a separate thread. Then, there's some notification via a shared socketpair because the main loop is around poll(). This actually doesn't work OOTB on OpenBSD due to a restrictive pledge(), but it's not a issue since sndio should be used there. libao is the last resort.