Modeled after std::filesystem::operator/() from C++17.

This is just the API design; there is no implementation yet, and no
caller.

This method was added in Boost 1.58.

Not updating m4/ax_boost_base.m4 because the new version breaks
cross-compilation by adding -I/usr/include to BOOST_CPPFLAGS.

Closes #296

Some Audio plugin (such as ALSA, and soon CoreAudio) already support microsecond level buffer time. However, current interval less than 1000 microseconds will cause a bug that the code treated as 0 ms.

Moving some text from the manpage.  Oh, documentation is still a mess.

Prepare for per-listener permissions.

some device seems to have issue with setting kAudioDevicePropertyVolumeScalar  with kAudioObjectPropertyElementMaster. Use AudioToolbox 's kAudioHardwareServiceDeviceProperty_VirtualMasterVolume instead. 

Ideally, we should get the steoro channels first, and set the kAudioDevicePropertyVolumeScalar for each channel, which is doable as presented in https://github.com/cmus/cmus/blob/master/op/coreaudio.c. I will do a follow up PR after refactor PR.

1 sec for pause is too long. we wait for the same amount of time as when ring buffer is not available for writing.

Remove sync_sample_rate option as it's no longer necessary

This PR will fix #271.

special thanks to @coroner21 who contributed a nice way to score hardware supported format in #292

Also, The DSD related code are all guarded with ENABLE_DSD  flag.

Fixes the NFS client build, closes #305.

- Update the mixer to set on device property instead of audio unit property. When user choose "hardware" as mixer type, they will be able to change the hardware device volume instead of the software (AudioUnit) volume. 
- We don't use square root scale in volume calculation as previous code did. This will make the volume level in line with system volume meter --- That is, MPD will have the same percentage volume reading compared to System Setting (Either in "System Preference" or in "Audio Midi Setup" app)