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.

Sometimes, it's useful to construct a DetachedSong from a LightSong
even without having real_uri initialized.

Eliminate one temporary allocation.

Mentioning these doesn't make a difference.

The "tagtypes" command now has several sub commands which can be used
to edit that mask.

handle_tagtypes() is also being moved to ClientCommands.cxx, because
that command will be extended to access client-specific settings.