audiod: fix enum of supported audio formats

audiod: fix enum of supported audio formats

Up to paraslash-0.2.12, audiod always supported every audio format since one
could use an external decoder and player as the stream write command. With the
single threaded audiod of 0.2.13 this is no longer possible, so make the enum
of supported audio formats dynamic, i.e. it only consists of those formats
for which suitable libs were found at compile time by configure.