Modeled after std::filesystem::operator/() from C++17.

Calling dbus_shutdown() too early will break the "udisks2" neighbor
plugin.

With Grand Central Dispatch used in Main.cxx, debug builds on macOS
crash as the IsInside() assertion gets triggered in the event loop. As
a simple fix, usage of GCD is removed. Plugging and unplugging
headphones or changes of the default output device was tested without
issues. Whatever the original commit tried to fix by GCD probably does
not need fixing anymore.

The normal I/O event thread can have a large latency, e.g. when
libgnutls loads all TLS CA certificates for a https connect.  This
makes it unreliable for the ALSA I/O notifications, and causes ring
buffer xruns.  To avoid interfering with high latency events such as
CURL's, we move the ALSA I/O events to a separate I/O thread which
also obtains real-time scheduling (if possible).

Closes #221

Fixup for commit 681e012f

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

Allows database plugins to use the EventThread, e.g. for CURL
integration.

Closes #34

The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

With this commit, multi-player support becomes possible... it's just
not wired to the frontend yet.

This is based on massive amounts of refactoring work I did over the
past 9 years.

Eliminate global variables.

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.

Eliminate dependency on io_thread_get().

Forward the configured ReplayGainMode to all outputs and decoders.

Yet another global variable removed.