Since we switched to C++ exceptions, there is no code path which
returns false.

We don't need to access Thread::handle early inside the EventThread,
so we don't need this trick anymore.

Eliminate global variables.

.. and not the input data.

Regression from commit b1c7649e (integer underflow).

 https://bugs.musicpd.org/view.php?id=4639

Use SND_PCM_NONBLOCK, and perform all snd_pcm_writei() calls in the
IOThread.  Use a lockless queue to copy data from the OutputThread to
the IOThread.

This rather major change aims to improve MPD's internal latency.  All
waits are now under MPD's control, instead of blocking inside
libasound2.

As a side effect, an output's filter is now decoupled from the actual
device I/O, which solves a major latency problem with the conversion
filter on slow CPUs and small period buffers.  See:

 https://bugs.musicpd.org/view.php?id=3900

The main EventLoop can block for a long time while a client's command
runs, and is therefore inappropriate for internal engine I/O.  This
fixes a serious regression for at least the "httpd" output, which used
to be hard-coded for the IOThread, but now receives the main EventLoop
as an initialization parameter.

For the mixers, this doesn't make much of a difference.  They are not
latency critical.

Fixes build breakage by commit 4e5271fc; and this method does make
sense in non-debug builds.

Fixes potential crash bug.

Now ClearSocketList() may only be called from PrepareSockets().
Calling it before destroying the object doesn't work properly, because
it doesn't unregister the TimeoutMonitor and the IdleMonitor.  Some of
its callers need to be fixed.

Change EventLoop::IsInside() call to EventLoop::IsInsideOrNull().
This means that BlockingCall() may be used during shutdown, after the
main EventLoop::Run() has finished.  This is important because mixers
are currently registered in the main EventLoop.

The libsidplayfp fork has merged libsidutils into the main library.
The libsidutils we used to link with was part of the original
libsidplay project.

Doesn't make a practical difference - but it's more correct this way.

Implement the second part of https://bugs.musicpd.org/view.php?id=3990

These days, this is common knowledge, and doesn't need a lengthy
explanation.

An explicit destructor prevents usage of implicit move operators, even
if it's empty.  Therefore, declaring a defaulted destructor with GCC
attribute "noinline" does what we want without preventing those
implicit operators.

This reverts commit 67b7d464.

Turns out I was wrong, and mentioning these does make a difference:
the implicit move constructor is not defined in the presence of a
user-declared destructor.

Some database plugins don't use a Storage (e.g. UPnP), and with this
plugin, DatabaseDetachSong() can crash.