Commit 3a212412 changed the meaning of out_audio_format, but I forgot
to apply this to ConvertFilter::FilterPCM().

See https://bugs.musicpd.org/view.php?id=4551

The latter doesn't exist, and this broke Vorbis-only builds.

See https://bugs.musicpd.org/view.php?id=4547

For easier state management inside filter plugins.

While this assert() was useful when we had to track the object's state
manually, there was no practical purpose other than verifying old
code, and it complicates our new C++ code.

The variable is actually used in this function.

This is a left-over from the previous version of the code
that was retrying on EINTR.

Some minor style fixes as well.

This can happen if you remove an external audio card or if you stop
sndiod(8) while playing a song.

sio_write() will retry internally if it fails with errno == EINTR
so no need to handle that.

It defaults to 250 ms.

Fixes build failure on Debian Wheezy.