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.

Obsoletes the same variable from AudioConfig.cxx.

Remove dependency on ReplayGain global variables.

Move code from replay_gain_get_real_mode().