Merge branch 'development'

Merge branch 'development'

Limit the maximum number of open file handles in Docker images.
Increase the level of optimisation (to -O3) while building the Shairport Sync application itself.
Enabled termination of a disconnected session in AirPlay 2 operation after a timeout, which was disabled by default.
Fixed a bug that prevented Shairport Sync from terminating cleanly when a fatal error that occurred while directly accessing the alsa output device.
Fixed a bug that prevented Shairport Sync from recovering cleanly if a player disconnected without warning. The problem was that the player thread would not respond to cancel request. Fixed by moving a pthreadtestcancel() call to the innermost loop.
Fixed compilation errors on old versions of Mac OS X by, reordering some of the files to be included in shairport.c.
Avoided using TCP_KEEPINTVL and TCP_KEEPCNT if they are not defined (they are not defined in older versions of Mac OS X).
Fixed a race condition with the metadata queues. The problem was that the queues were being initialised by threads launched by the main thread which, having started the threads, proceeded to use the queues. But if the threads were late in starting, the queues might not be initialised by the time the main thread tried to use them. Fixed by initialising the queues in the main thread.
Enable the ALSA backend to access mixers on a devices with a hdmi: prefix.
Update the help text for the ALSA backend to denote HDMI devices using the hdmi: prefix rather than hw:.
Manage thread cancellation state explicitly in audio_ao.c
CAR INSTALL guide: Simplify some of the wording. Change order and allow a few seconds before starting systemd-timesync service.