Prepare to reuse it in another class.

ALSA is just one out of many output plugins, and detailed plugin
documentation should only live in the user manual, without having
duplicates in the (brief) manpage.

Also move "mixer_type" to the "optional audio output parameters"
section; it is a generic option, not specific to ALSA.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/552

The HugeArray constructor can throw std::bad_alloc, and so can
SparseBuffer's constructor.

On Windows, we keep using our own implementations, because GCC
implements std::mutex and std::condition_variable with pthread
emulation, which is not a good choice.

Prepare the transition to std::mutex.

GetChromaprintCommand::DecodeStream() is called without holding the mutex.