Remove ->fd of struct audio file data.

Remove ->fd of struct audio file data.

This structure contains information about the next audio file. It
is stored in a shared memory area, and a reference to this area is
sent through a pipe from the afs process to the server process. The
file descriptor of the next audio file, however, must be passed via
Unix socket magic (SCM_RIGHTS) and thus does not need to be part of
the structure.

Moreover, it's easier to define the afd structure in
open_and_update_audio_file() of aft.c rather than in its caller,
open_next_audio_file() of afs.c, because the caller only needs the
fd of the audio file and the shared memory ID but not the audio file
data structure itself.

Expand the documentation of open_and_update_audio_file() a bit while
at it.