Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Coverity discovered that the Pulse plugin could throw exceptions from
Pause(), but that method was marked "noexcept" because its caller was
not designed to catch exceptions.  So instead of avoiding exceptions
(by catching and logging them in each and every implementation), let's
allow them, and do the catch/log game in the MPD core.

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.

struct AudioOutput should not know or care about multi-threading.

Fixes build failure on OS X, closes #44.  With the other plugins,
that's not critical, because those use the AudioOutputWrapper, which
hides this problem.

The AudioOutput struct (which is exposed to all plugins) should not be
aware that it's being controlled by another thread.

Eliminate dependency on io_thread_get().

The old struct config_param remains only for top-level string options.