au: validate bits-per-sample separately from codec tag
au: set bit rate
au: set block_align and use it in au_read_packet()
au: do not set pkt->size directly

It is already set by av_get_packet() even for partial reads.
au: do not arbitrarily limit channel count

Nothing in the AU specification sets a limit on channel count.
We only need to avoid an overflow in the packet size calculation.
au: move skipping of unused data to before parameter validation

Also do not unnecessarily skip 0 bytes.
au: validate sample rate
au: use %u when printing id and channels since they are unsigned
au: set stream start time and packet durations
au: use ff_raw_write_packet()
au: cosmetics: pretty-print and remove pointless comments
au: return AVERROR codes instead of -1
au: remove unnecessary casts