This fixes an old bug which caused the "unused" warnings to be
unreliable; only the first block in the list was marked as being
"used", no matter if it was really used, and the rest was never marked
as "used", suppressing all warnings for them.

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.