Avoid implicit copy.

This is the beginning of multi-player support.  There will be support
for multiple Partition objects in one MPD process.

Push the new mode to the filter instead of accessing global variables
through replay_gain_get_real_mode().

replay_gain_get_real_mode() accesses dangerous globals.  Let's just
use the global variable replay_gain_mode, and assume "AUTO" means
"TRACK".  At this point, this is the best we can do, because neither
the decoder nor the player should know about the current replay gain
mode.  This is applied as a filter in the output thread.

Let the function decoder_replay_gain() update
decoder_control::replay_gain_db instead of letting each decoder plugin
take care for that.

Don't use the global variable "g_playlist".

Reduce access to the global variable "g_playlist".

Move the configuration lookup to Main.cxx.

Use the new HugeAllocator as backend for SliceBuffer and call
HugeDiscard() when the last chunk was returned.

Avoid page faults on MPD startup.  This saves a lot of memory for an
idle MPD.

This is harmful for memory usage, because it forces the kernel to
allocate physical memory, even before playback has started.  No bug
has been found in a few years with this, so it's safe to assume that
this code is not necessary.

Random access is not necessary here.

.. instead of rolling own classes.