major afs cleanup

This patch  makes afs.c the only user of the audio format handler
arry afl[] in order to completely separate audio format handling from
audio file sending.

This is achieved by providing the new helper functions afs_get_header()
and afs_chunk_time(). audio file senders may call these functions
without knowledge of struct audio_format. This allows to get rid of
the "af" parameter for the pre_select and post_select functions of
struct sender.

Related to this cleanup is the removal of match_audio_file_name() from db.c
which also depended on afl[]. This function was essentially identical to
afs.c's guess_audio_format(), so make the latter public and use it from
within db.c.