This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

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

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

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.

Fixes use-after-free bug (https://bugs.musicpd.org/view.php?id=4548).

Comes with a callback pointer instead of a virtual method, which
allows multiple instances in one class.

Replaces the old BoundMethod template.

bind fails with an "address already in use" error if the socket path
already exists.

This was broken by cbb595ba.

Manages memory automatically.