Keep MPD's footprint small until playback is requested.

Eliminates unnecessary mutex locks.

When the decoder is still starting up while we handle a SEEK, finish
the "player SEEK" immediately and re-enter the player loop, being able
to handle commands (and even cancel the pending seek).

This is the first part in a series of patches to solve the "blocking
input blocks decoder, blocks player, blocks the main thread" problem.
There are many other blocking code locations left, and the main thread
isn't non-blocking either because it waits for "seeking" to become
false.

Reduces main thread contention.  Avoids blocking the main thread in
"status" commands.

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

Eliminate a dependency from MultipleOutputs on PlayerControl.

Obsoletes the same variable from AudioConfig.cxx.

Don't use the global variables in class DecoderBridge; instead,
forward these values to the decoder thread via PlayerControl and
DecoderControl.

Prepare full C++ exception support in the player thread.