Look up top-level config options by enum (= integer), not by name
string.

Fixes crash when another object attempts to access the EventLoop
during destruction.

The main_cond variable was completely unnecessary.  The pc.cond object
can be used for both main->pc and pc->main.

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().

Move the configuration lookup to Main.cxx.