Switch to the new afs.

This obsoletes get_audio_file() of vss.c. It is being replaced
by a call to the afs layer from vss_post_select() which requests
a struct audio_format_data for the next audio file.

The afd struct, the chunk table and the path of the new audio file are
stored in a shared memory area. The id of that area is sent through
the afs-server socket. An open fd for the underlying audio file is
send to the server process as well using usual socket magic.

The vss task of the server process attaches the shared memory area
and mmaps the open fd to start audio streaming.

The code is still quite buggy, but let's do bug fixes and removal of
the old audio file selectors in susequent patches.