Abolish uaudio_apis[]. Instead, define UAUDIO_DEFAULT to indicate

Abolish uaudio_apis[].  Instead, define UAUDIO_DEFAULT to indicate
default sound API.  This saves e.g. uaudio_rtp being linked into
disorder-playrtp.  It does mean that users need more knowledge of
available APIs, but since they need to know what options to set that's
not much of an extra burden.