RoarAudio's sndio emulation has been a source for annoyances.  First,
their headers turned out to be broken with C++, due to their use of
the "new" keyword.  Then they used a preprocessor macro to rename
"sio_hdl" to something else, effectively disallowing the use of
forward declarations.  Enough is enough, and I'm removing support for
it.

RoarAudio users should better use the RoarAudio output plugin.

no change in behaviour.

Yet another C-style vtable replaced with C++.

Hide struct FilteredAudioOutput from the plugins, preparing for hiding
MPD's core internals.

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

Same as in commit e02d8ad8, but this time for the sndio plugin
which can be emulated by libroar.

Eliminate dependency on io_thread_get().

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.