The CurlRequest gives us a more low-level API to CURL without the
InputStream interface, integrated into our IOThread.

The user unit omits the "ProtectKernelModules" setting which fails
with modular kernels:

 Failed at step CAPABILITIES spawning /usr/bin/mpd: Operation not permitted

It is unfortunate that systemd (version 232) is unable to reduce its
own capabilities, because this requires us to split system and user
units.

 https://bugs.musicpd.org/view.php?id=4608

Obsoletes the same variable from AudioConfig.cxx.

Remove dependency on ReplayGain global variables.

Move code from ReplayGainConfig.cxx.

Speed up container_scan() again, by eliminating the need to call
scan_file() for each item.

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

Make all the locks exception-safe.

Reduce header clutter.

Similar to boost::scope_exit, but fewer include dependencies.