aout_PacketInit() does not allocate anything yet, but it will have too.

By the way, the error paths of the (untouched by the commit)
DirectSound plugin seem a bit suspicious to me...

This provides aout_PacketPlay(), aout_PacketPause() and
aout_PacketFlush() helpers for legacy audio outputs. They conveniently
match the callback prototypes of pf_play, pf_pause and pf_flush
respectively.

Unfortunately, this will break S/PDIF pass-through for DTS if HAVE_FPU
is false, just like A/52 and MPEG Audio pass-through are broken if
a52dec or libmad are absent.

Bits per sample only make no sense for the output.

Putting these in pkg-config is a bit of ugly, but it makes sense since
we only build a static library. As a reference, upstream libav does the
exact same thing already.